NERVOUS SYSTEM OF PILA
Nervous system of Pila consists of paired ganglia, commissures and connectives uniting them, and nerves running from these central organs to all parts of the body.
1. Ganglia: Paired ganglia which are groups of nerve cells are as follows.
a) Cerebral ganglia: A pair of roughly triangular ganglia, situated anteriorly on the dorso-lateral sides of the buccal mass.
b) Buccal ganglia: A pair of small, triangular ganglia, lying dorso-laterally one on either side at the junction of the buccal mass and oesophagus, partly embedded in the muscles.
c) Pleuro-pedal ganglia: A pair of large, somewhat triangular ganglionic masses present one on either ventro-lateral side of the buccal mass.
Each one is formed by the fusion of an outer pleural and an inner pedal ganglion, separated by a faint notch.
Right pleuro-pedal ganglionic mass also consists of the infra-intestinal ganglion fused with it.
d) Supra-intestinal ganglion: An unpaired fusiform ganglion, lying in a sinus behind the left pleuro-pedal ganglionic mass.
e) Visceral ganglia: A single ganglionic mass representing two fused ganglia, situated at the lower end of the visceral mass.
2. Commissures: Commissures are those nerves which establish connections between two similar ganglia and lying dorsally to the buccal mass.
b) Buccal commissure: A fine nerve which connects the two buccal ganglia and runs transversely on the ventral side of the oesophagus.
c) Pedal commissures: Two thick nervous bands that lie one above the other underneath the buccal mass and connect the two pedal ganglia together.
3. Connectives: Connectives are those nerves which connect two different ganglia. In the nervous system of Pila, these are
a) Two cerebro-buccal connectives: These connect, on either side, the cerebral ganglion and buccal ganglion together.
b) Two cerebro-pleural connectives: These connect, on either side, the cerebral and outer pleural ganglion of the pleuro-pedal ganglionic mass.
c) Two cerebro-pedal connectives: These connect, on either side, the cerebral and inner pedal ganglion of the pleuro-pedal ganglionic mass.
d) Pleuro-infra intestinal connective: Also called infra-intestinal nerve, it is a nerve connection between the pleural ganglion of the left pleuro-pedal mass and the infra-intestinal ganglion which is fused with the right pleuro-pedal mass.
e) Infra-intestinal visceral connective: Running below the intestine, it is a long nerve that connects the visceral ganglion with the infra-intestinal part of the right pleuro-pedal-infra-intestinal ganglionic mass.
f) Supra-intestinal visceral connective: Running above the intestine, it is a slender nerve that connects the visceral ganglion with the supra-intestinal ganglion.
g) Supra-intestinal-pleural connective: Also called the supra-intestinal nerve, it connects the supra-intestinal ganglion with the pleural part of the right pleuro-pedal-infra-intestinal ganglionic mass.
h) Zygoneury: It is a nerve connection between the pleural part of the left pleuropedal ganglionic mass and supra-intestinal ganglion.
4. Nerves to different parts of the body: Various ganglia send nerves to innervate different parts of the body
i) Each cerebral ganglion gives off nerves, supplying the snout, skin, tentacle and buccal mass anteriorly and the tentacle, eye and statocyst, posteriorly.
ii) Buccal ganglion of each side , sends nerves to innervate the buccal mass, radular sac, salivary glands, oesophagus and oesophageal pouches
iii) Pedal ganglia gives off nerves, anteriorly as well as posteriorly, to innervate the foot. Statocyst, on each side, is also connected, by a band of, connective tissue, to each pedal ganglion.
iv) Left pleural ganglion innervates the parietal wall, mantle, osphradium, left nuchal lobe, columellar muscle and anterior part of the gill.
v) Pight pleural ganglion innervates the parietal wall, epitaenia, right nuchal lobe, copulatory organ, columellar muscle and rectum.
vi) Supra-intestinal ganglion gives off a stout nerve to innervate the mantle and the anterior part of ctenidium, while its connective with the left pleural ganglion sends a few nerves to the parietal wall.
vii) Nerves from the visceral ganglion supply the renal organ, genital organs, pericardium, stomach, intestine, digestive gland etc.
Sense organs
A snail is diffusely sensitive for the sensory cells are distributed all over the head, foot and various other parts of the body. Special sense organs of sense include a single osphradium and paired eyes, statocysts, labial palps and tentacles.
I) Osphradium: Single osphradium is situated on the left side of the animal suspended from the roof of the mantle cavity close to the entrance through the left nuchal lobe.
It is a small, elongated, oval structure, about 6-7 mm long. While broadest in the middle, its inner or right end is bluntly rounded and the outer or left end is somewhat pointed.
It is bipectinate consisting of 22-28 thick, fleshy, and roughly triangular leaflets, arranged in two rows along a slightly raised median or central axis.
Leaflets are largest in the middle of the osphradium. Each leaflet is attached to the mantle wall by its broad base, to the central axis by its smaller inner side, while its outer longer side remains free.
Osphradium is supplied by a nerve from the left pleural ganglion.
In a transverse section, the osphradium consists of an outermost covering of a single layered epithelium, internally lined by a thin basement membrane, the interior filled up with nerves, connective tissue and blood spaces.
Epithelial cells are elongated possessing basal nuclei, and they are of three types
i) sensory
ii) ciliated
iii) glandular
The ciliated cells line the attached margin, while the sensory cells are with out cilia, cover the osphradium. Flask-shaped glandular cells are found scattered among the sensory cells.
Osphradium hangs like a curtain in the path of the respiratory water current, and probably serves as an olfactory organ. Its name has been derived from a Greek word, meaning to smell.
It serves to test the chemical nature of the inspiratory water current. In case the water is foul, its entry into the mantle cavity is stopped by the closure of the left nuchal lobe.
It may also help in the selection of the food material.
II) Eyes:
Snail’s head carries a pair of short fleshy and stump-like stalks or ommatophores, one on either side, behind the second pair of tentacles.
Each ommatophore bears a small, black and circular eye, slightly below its tip on the outer side.
In spite of their elaborate structure, eyes of Pila are probably not true organs of sight. Sense of sight is greatly limited in range and the snail does not seem to distinguish objects, but only responds to changes in the intensities of light and detect quick movements.
Most of the snails feed at night probably because their eyes are adapted to dim light. In some snails the lost eyes can be replaced by regeneration, a process which has been recorded to occur 20 times in succession.
III) Statocysts:
Statocysts are a pair of small, pyriform and cream-coloured sacs, lying one on either side attached to the pedal ganglion of that side by a band of connective tissue.
Each statocyst lies in a depression, posterior and outer to the ganglion. Each statocyst is a hollow capsule surrounded by an outer thick, tough, leathery covering of connective tissue.
Wall of the capsule is made of a single layer of ectodermal cells, and supplied by a nerve from the cerebral ganglion.
Cavity of the capsule is filled with a fluid and a variable number of minute, oval and calcareous particles the statoconia. Statocysts are organs of equilibrium.
IV) Tentacles
The snout of Pila is anteriorly prolonged into a pair of short, contractile and conical processes bordering the mouth.
These are the labial palps or the anterior or first pair of tentacles.
Behind them arise, one on either side, a pair of long, tapering filamentous and highly contractile whip-like processes, the true or second pair of tentacles.
Tentacles are of the same colour as the snout and tactile in function. A sense of taste is doubtfully attributed to the labial palps.
Zoology in the Classroom - is a blog for teachers and students of zoology. I have been teaching as Zoology teacher for the last 30 years. I post the notes or handouts that I supply to my students in my classroom. Hope this will benefit Zoology fraternity
Saturday, July 29, 2017
Thursday, July 13, 2017
PRACTICAL NOTES
1. Eleutheronema tetradactylum
Kingdom: Animalia
Phylum: Chordata
Class: Actinopterygii
Order: Perciformes
Family: Polynemidae
Genus: Eleutheronema
Species: tetradactylum
1. Eleutheronema tetradactylum, also known as Indian Salmon or Rawas, is a threadfin fish of the Polynemidae family.
2. This highly commercial fish known for uses in aquaculture occur mainly over shallow muddy bottoms in coastal waters forming loose schools.
3. Adults of this highly vulnerable species enter rivers during winter.
4. Adults feed on prawns and fish with occasional polychaetes, while juveniles feed on prawns shrimps and mysids.
2. Epinephelus malabaricus
Kingdom: Animalia
Phylum: Chordata
Class: Actinopterygii
Order: Perciformes
Family: Serranidae
Genus: Epinephelus
Species: malabaricus
1. The Malabar grouper is widespread throughout the tropical waters of the Indo-West Pacific area from the eastern coast of Africa to the Tonga Islands, Red Sea included.
2. This grouper lives in various habitats, such as lagoons, mangroves, coral and rocky reefs, sandy and muddy bottom areas, between 2 and 150 m deep.
3. The juveniles prefers lagoon or brackish areas.
4. It has a light grey to light brownish background color, with a number of dark brown spots randomly scattered.
5. The body has also a various number of brown diagonal stripes, but in maturity they seem to become a uniform darker colour.
6. Young fish have numerous brown spots. The tail fin is rounded.
Kingdom: Animalia
Phylum: Chordata
Class: Actinopterygii
Order: Clupeiformes
Family: Clupeidae
Subfamily: Alosinae
Genus: Hilsa
Species: ilisha
3. Hilsa ilisha
1. It is found in rivers and estuaries in India, Pakistan, Bangladesh, Burma and the
2. It has no dorsal spines but 18 - 21 Dorsal soft rays and anal soft rays. The belly has 30 to 33 scutes.
3. There is a distinct median notch in upper jaw.
4. Gill rakers fine and numerous, about 100 to 250 on lower part of arch and the fins are hyaline.
5. The fish shows a dark blotch behind gill opening, followed by a series of small spots along the flank in juveniles.
4. The species filter feeds on plankton and by grubbing muddy bottoms.
4. Labeo calbasu
Kingdom: Animalia
Phylum: Chordata
Class: Actinopterygii
Order: Cypriniformes
Family: Cyprinidae
Genus: Labeo
Species: calbasu
1. Labeo is a genus of carps in the family Cyprinidae. They are found mainly in the Old World tropics.
2. It contains the typical labeos in the subfamily Labeoninae, which may not be a valid group, however, and is often included in the Cyprininae as tribe Labeonini.
3. The labeos appear fairly similar to the "freshwater sharks" of the genus Epalzeorhynchos, which is also part of the Labeoninae (or Labeonini), but is not very closely related.
4. Labeos are larger, and have a more spindle-shaped body, as they are mostly free-swimming rather than benthic like Epalzeorhynchos.
5. Their mouths look very different, too; they have a pronounced rostral cap, which covers the upper lip except when feeding.
6. The lips are expanded into thick, sausage-shaped pads which have keratinized edges.
5.Megalops cyprinoides
Kingdom: Animalia
Phylum: Chordata
Class: Actinopterygii
Order: Elopiformes
Family: Megalopidae
Genus: Megalops
Species: cyprinoides
1. The Indo-Pacific tarpon, Megalops cyprinoides, also known as the Oxeye herring or simply herring, is a relatively medium-sized species of tarpon.
2. In appearance, it is like the Atlantic tarpon, Megalops atlanticus: olive-green on top, and silver on the sides.
3. The large mouth is turned upwards; the lower jaw contains an elongated, bony plate.
4. The last ray of the dorsal fin is much longer than the others, reaching nearly to the tail.
5. It is capable of filling its swim bladder with air and absorbing oxygen from it.
6. Species in fresh water tend to be smaller than the saltwater species, growing just over 50 centimetres (20 in), while saltwater species grow over a 1 metre (3.3 ft).
6. They are an opportunistic feeder, feeding on smaller fish, crustaceans, and evenplants rarely.
Kingdom: Animalia
Phylum: Chordata
Class: Actinopterygii
Order: Perciformes
Family: Carangidae
Genus: Parastromateus
Species: niger
6.Parastromateus niger
1. The black pomfret, Parastromateus niger, is a species of carangid native to reefs of the Indian Ocean and the western Pacific Ocean.
2. It is found at depths from 15 to 105 m (49 to 344 ft), though it is rarely found deeper than 40 m (130 ft).
3. This species grows to 75 cm (30 in) in total length and is very important to local commercial fisheries. This species is the only known member of its genus.
Kingdom: Animalia
Phylum: Chordata
Class: Actinopterygii
Order: Perciformes
Family: Scombridae
Genus: Scomberomorus
Species: commerson
7. Scomberomorus commerson
1. They are vivid blue to dark grey in colour along their backs and flanks and fade to a silvery blue-grey on the belly.
2. Spanish mackerel have scores of narrow, vertical lines down their sides.
3. Spanish mackerel are the largest of all Australian mackerels, growing to about 200 cm and up to 70 kg.
4. Spanish mackerel spawn in oceanic conditions on reef edges.
5. Eggs have a large oil droplet that aids in buoyancy and keeps them at the top of the water column which is warmer, well oxygenated, and has an abundant planktonic food supply for the larvae once they are hatched.
8. Stromateus argenteus
Kingdom: Animalia
Phylum: Chordata
Class: Actinopterygii
Order: Perciformes
Family: Stromateidae
Genus: Pampus
Species: argenteus
1. Pampus argenteus, often called either the silver or white pomfret, is a species of butterfish that lives in coastal waters off the Middle East, South Asia, and Southeast Asia.
2. Fish of this family are characterized by their flat bodies, forked tail fins, and long pectoral fins.
3. Silver pomfrets are usually silver/white in color, with few small scales.
4. They can grow up to 4-6 kg. However, due to overfishing, specimens weighing less than 1 kg are more commonly seen.
5. It is called pamplet in Mumbai.
Tilapia mossambica
Phylum: Chordata
Class: Actinopterygii
Order: Perciformes
Family: Cichlidae
Genus: Tilapia
Species: mossambicus
1. The Mozambique tilapia, Oreochromis mossambicus, is a tilapiine cichlid fish native to southern Africa.
2. It is a popular fish for aquaculture. Dull colored, the Mozambique tilapia often lives up to a decade in its native habitats.
3. This makes it an optimal species for aquaculture because it readily adapts to new situations. It is known as Black Tilapia in Colombia and as Blue Kurper in South Africa.
4. The native Mozambique tilapia is laterally compressed, and has a deep body with long dorsal fins, the front part of which have spines.
5. It is a remarkably robust and fecund fish, readily adapting to available food sources and breeding under suboptimal conditions.
Phylum: Chordata
Class: Actinopterygii
Order: Perciformes
Family: Rachycentridae
Genus: Rachycentron
Kaup, 1826
Species: R. canadum
Rachycentron canadum
1. The cobia (Rachycentron canadum) is a species of perciform marine fish, the only representative of the genus Rachycentronand the family Rachycentridae.
2. The another common names include black kingfish, black salmon, ling, lemonfish, crabeater,prodigal son and aruan tasek.
3. Attaining a maximum length of 2 m (78 in) and maximum weight of 78 kg (172 lb), the cobia has an elongated fusiform (spindle-shaped) body and a broad, flattened head.
4. The eyes are small and the lower jaw projects slightly past the upper. Fibrous villiform teeth line the jaws, the tongue, and the roof of the mouth.
5. The body of the fish is smooth with small scales. It is dark brown in color, grading to white on the belly with two darker brown horizontal bands on the flanks.
Scomberomorus guttatus
Phylum: Chordata
Class: Actinopterygii
Order: Perciformes
Family: Scombridae
Genus: Scomberomorus
Species: guttatus
1. Indo-Pacific king mackerel or popularly (spotted) seer fish (Scomberomorus guttatus) is a sea fish among the mackerelvariety of fishes. It is found in around the Indian ocean and adjoining seas.
2. It is a popular game fish and grows up to 45 kg (100 lbs) and is a strong fighter, that has on occasion been seen to leap out of the water when hooked.
3. It is very popular among the countries of the Indian subcontinent including India, Sri Lanka and Bangladesh. It's a fairly expensive fish that's considered a delicacy in most places.
Kingdom: Animalia
Phylum: Chordata
Class: Actinopterygii
Order: Perciformes
Family: Polynemidae
Genus: Eleutheronema
Species: tetradactylum
1. Eleutheronema tetradactylum, also known as Indian Salmon or Rawas, is a threadfin fish of the Polynemidae family.
2. This highly commercial fish known for uses in aquaculture occur mainly over shallow muddy bottoms in coastal waters forming loose schools.
3. Adults of this highly vulnerable species enter rivers during winter.
4. Adults feed on prawns and fish with occasional polychaetes, while juveniles feed on prawns shrimps and mysids.
2. Epinephelus malabaricus
Kingdom: Animalia
Phylum: Chordata
Class: Actinopterygii
Order: Perciformes
Family: Serranidae
Genus: Epinephelus
Species: malabaricus
1. The Malabar grouper is widespread throughout the tropical waters of the Indo-West Pacific area from the eastern coast of Africa to the Tonga Islands, Red Sea included.
2. This grouper lives in various habitats, such as lagoons, mangroves, coral and rocky reefs, sandy and muddy bottom areas, between 2 and 150 m deep.
3. The juveniles prefers lagoon or brackish areas.
4. It has a light grey to light brownish background color, with a number of dark brown spots randomly scattered.
5. The body has also a various number of brown diagonal stripes, but in maturity they seem to become a uniform darker colour.
6. Young fish have numerous brown spots. The tail fin is rounded.
Kingdom: Animalia
Phylum: Chordata
Class: Actinopterygii
Order: Clupeiformes
Family: Clupeidae
Subfamily: Alosinae
Genus: Hilsa
Species: ilisha
3. Hilsa ilisha
1. It is found in rivers and estuaries in India, Pakistan, Bangladesh, Burma and the
2. It has no dorsal spines but 18 - 21 Dorsal soft rays and anal soft rays. The belly has 30 to 33 scutes.
3. There is a distinct median notch in upper jaw.
4. Gill rakers fine and numerous, about 100 to 250 on lower part of arch and the fins are hyaline.
5. The fish shows a dark blotch behind gill opening, followed by a series of small spots along the flank in juveniles.
4. The species filter feeds on plankton and by grubbing muddy bottoms.
4. Labeo calbasu
Kingdom: Animalia
Phylum: Chordata
Class: Actinopterygii
Order: Cypriniformes
Family: Cyprinidae
Genus: Labeo
Species: calbasu
1. Labeo is a genus of carps in the family Cyprinidae. They are found mainly in the Old World tropics.
2. It contains the typical labeos in the subfamily Labeoninae, which may not be a valid group, however, and is often included in the Cyprininae as tribe Labeonini.
3. The labeos appear fairly similar to the "freshwater sharks" of the genus Epalzeorhynchos, which is also part of the Labeoninae (or Labeonini), but is not very closely related.
4. Labeos are larger, and have a more spindle-shaped body, as they are mostly free-swimming rather than benthic like Epalzeorhynchos.
5. Their mouths look very different, too; they have a pronounced rostral cap, which covers the upper lip except when feeding.
6. The lips are expanded into thick, sausage-shaped pads which have keratinized edges.
5.Megalops cyprinoides
Kingdom: Animalia
Phylum: Chordata
Class: Actinopterygii
Order: Elopiformes
Family: Megalopidae
Genus: Megalops
Species: cyprinoides
1. The Indo-Pacific tarpon, Megalops cyprinoides, also known as the Oxeye herring or simply herring, is a relatively medium-sized species of tarpon.
2. In appearance, it is like the Atlantic tarpon, Megalops atlanticus: olive-green on top, and silver on the sides.
3. The large mouth is turned upwards; the lower jaw contains an elongated, bony plate.
4. The last ray of the dorsal fin is much longer than the others, reaching nearly to the tail.
5. It is capable of filling its swim bladder with air and absorbing oxygen from it.
6. Species in fresh water tend to be smaller than the saltwater species, growing just over 50 centimetres (20 in), while saltwater species grow over a 1 metre (3.3 ft).
6. They are an opportunistic feeder, feeding on smaller fish, crustaceans, and evenplants rarely.
Kingdom: Animalia
Phylum: Chordata
Class: Actinopterygii
Order: Perciformes
Family: Carangidae
Genus: Parastromateus
Species: niger
6.Parastromateus niger
1. The black pomfret, Parastromateus niger, is a species of carangid native to reefs of the Indian Ocean and the western Pacific Ocean.
2. It is found at depths from 15 to 105 m (49 to 344 ft), though it is rarely found deeper than 40 m (130 ft).
3. This species grows to 75 cm (30 in) in total length and is very important to local commercial fisheries. This species is the only known member of its genus.
Kingdom: Animalia
Phylum: Chordata
Class: Actinopterygii
Order: Perciformes
Family: Scombridae
Genus: Scomberomorus
Species: commerson
7. Scomberomorus commerson
1. They are vivid blue to dark grey in colour along their backs and flanks and fade to a silvery blue-grey on the belly.
2. Spanish mackerel have scores of narrow, vertical lines down their sides.
3. Spanish mackerel are the largest of all Australian mackerels, growing to about 200 cm and up to 70 kg.
4. Spanish mackerel spawn in oceanic conditions on reef edges.
5. Eggs have a large oil droplet that aids in buoyancy and keeps them at the top of the water column which is warmer, well oxygenated, and has an abundant planktonic food supply for the larvae once they are hatched.
8. Stromateus argenteus
Kingdom: Animalia
Phylum: Chordata
Class: Actinopterygii
Order: Perciformes
Family: Stromateidae
Genus: Pampus
Species: argenteus
1. Pampus argenteus, often called either the silver or white pomfret, is a species of butterfish that lives in coastal waters off the Middle East, South Asia, and Southeast Asia.
2. Fish of this family are characterized by their flat bodies, forked tail fins, and long pectoral fins.
3. Silver pomfrets are usually silver/white in color, with few small scales.
4. They can grow up to 4-6 kg. However, due to overfishing, specimens weighing less than 1 kg are more commonly seen.
5. It is called pamplet in Mumbai.
Tilapia mossambica
Phylum: Chordata
Class: Actinopterygii
Order: Perciformes
Family: Cichlidae
Genus: Tilapia
Species: mossambicus
1. The Mozambique tilapia, Oreochromis mossambicus, is a tilapiine cichlid fish native to southern Africa.
2. It is a popular fish for aquaculture. Dull colored, the Mozambique tilapia often lives up to a decade in its native habitats.
3. This makes it an optimal species for aquaculture because it readily adapts to new situations. It is known as Black Tilapia in Colombia and as Blue Kurper in South Africa.
4. The native Mozambique tilapia is laterally compressed, and has a deep body with long dorsal fins, the front part of which have spines.
5. It is a remarkably robust and fecund fish, readily adapting to available food sources and breeding under suboptimal conditions.
Phylum: Chordata
Class: Actinopterygii
Order: Perciformes
Family: Rachycentridae
Genus: Rachycentron
Kaup, 1826
Species: R. canadum
Rachycentron canadum
1. The cobia (Rachycentron canadum) is a species of perciform marine fish, the only representative of the genus Rachycentronand the family Rachycentridae.
2. The another common names include black kingfish, black salmon, ling, lemonfish, crabeater,prodigal son and aruan tasek.
3. Attaining a maximum length of 2 m (78 in) and maximum weight of 78 kg (172 lb), the cobia has an elongated fusiform (spindle-shaped) body and a broad, flattened head.
4. The eyes are small and the lower jaw projects slightly past the upper. Fibrous villiform teeth line the jaws, the tongue, and the roof of the mouth.
5. The body of the fish is smooth with small scales. It is dark brown in color, grading to white on the belly with two darker brown horizontal bands on the flanks.
Scomberomorus guttatus
Phylum: Chordata
Class: Actinopterygii
Order: Perciformes
Family: Scombridae
Genus: Scomberomorus
Species: guttatus
1. Indo-Pacific king mackerel or popularly (spotted) seer fish (Scomberomorus guttatus) is a sea fish among the mackerelvariety of fishes. It is found in around the Indian ocean and adjoining seas.
2. It is a popular game fish and grows up to 45 kg (100 lbs) and is a strong fighter, that has on occasion been seen to leap out of the water when hooked.
3. It is very popular among the countries of the Indian subcontinent including India, Sri Lanka and Bangladesh. It's a fairly expensive fish that's considered a delicacy in most places.
Monday, July 10, 2017
POLYMORPHISM IN COELENTERATES
POLYMORPHISM IN COELENTERATES
The presence of polymorphism in cnidarians is one of their characteristic features. It is defined as the occurrence of structurally and functionally different types of individuals within the same organism during its life cycle.
A species that exhibits this phenomenon is called polymorphic.
Polymorphism is predominantly exhibited by the different animals of class- hydrozoa.
Hydroid colonies which bear two types of zooids are known as dimorphic, while colonies which bear more than two types of zooids are called polymorphic colonies.
Two basic forms
In Hydrozoa (or Coelenterates), which may be single or colonial, there occur two main types of individuals or zooids- polyps and medusae.
1. Polyp: It is sessile (fixed to the substratum) with a hydra like body attached to the main colony by narrower end. Its free end is wider and raised into hypostome that bears a mouth surrounded by tentacles. It faces upwards and carries the function of feeding the colony. Polyps are specialized for feeding and thus known as gastrozooids.
2. Medusa: It is a free swimming zooid with an umbrella shaped body having exumbrellar and subumbrellar surfaces. It has a mouth facing downwards (present on the tubular growth called manubrium hanging down from subumbrellar surface) in contrast to polyp in which mouth faces upwards.
Like hydranth, it can feed on its own for its survival until its function is over. It bears four gonads (testis or ovary) which produces either sperms or ova at the time of maturity and is responsible for sexual reproduction therefore also known as gonozooid. They normally die after reproducing the colony. Therefore, they not only help in sexual reproduction but also play an important role in dispersal of the colony.
Importance of Polymorphism
Polymorphism is essentially a phenomenon of division of labour. Different functions are assigned to different individuals, rather than to parts or organs of one individual.
Thus, polyps are concerned with feeding, protection and asexual reproduction, while medusae are concerned with sexual reproduction.
Patterns of polymorphism
Degree of polymorphism varies greatly in different groups of Hydrozoa
1. Dimorphic. Simplest and commonest pattern of polymorphism is exhibited by many hydrozoan colonies like Obelia, Tubularia, Campanularia etc.
They have only two types of zooids. Gastrozoids or hydranths are concerned with feeding. While gonozoids or blastostyles with asexual budding forming sexual medusae or gonophores.
Such colonies, bearing only two types of individuals are called dimorphic and the phenomenon is termed dimorphism.
2. Trimorphic. Some forms like Plumularia, are trimorphic. Besides Gastrozooids and gonozooids, they also possess a third type of individuals, the dactylozooids. These are functionally non-feeding and defensive polyps bearing batteries of nematocysts.
3. Polymorphic. Coelenterates having more than three types of individuals are called polymorphic.
A somewhat greater degree of polymorphism is found in the encrusting colony of Hydractinia with five types of polyps each performing a specialized function.
These are: 1. Gastrozooids for feeding
2. Spiral dactylozooids for protection
3. long sensory tentaculozooids with sensory cells
4. skeletozooids as spiny projections of chitin
5. gonozooids or reproductive individuals, bearing male or female gonophores or medusae for sexual reproduction
Extreme examples of polymorphism are seen in the pelagic or swimming colonies of the orders
Siphonophora (Diphes, Halistemmia, Stephalia, physalia) and Chondrophora (porpita, velella).
Polymorphis reaches its peak in siphonophora.
(a) Modifications of polyps
Polyps structurally get modified into different types of zooids according to the requirement of an individual, which are described below:
i. Gastrozooids: These are feeding zooids and resemble the structure of polyp without usual tentacles. They are tubular, elongated, with a mouth facing towards the bottom of the colony.
ii. Dactylozooids: These are protective zooids. They may be also called as feelers or palpons. Structurally, they look like gastrozooids but are blind structures without any mouth.
iii. Gonozooids: They may resemble gastrozooids having mouth but are without tentacles and bear medusa. In other the gonozooids may form stalked branches bearing grape like structures called gonophores. Sometimes tentacles like dactylozooids are attached to them which are called gonopalpons.
iv.Pneumatophore:
It is a hydrostatic apparatus present in siphonophores. It is gas filled chamber that appear to be a highly modified polyps (although previously considered as derived from medusae).
It helps in keeping the body in an upright condition while floating.
It is without mesogloea but the umbrella cavity contains an air chamber called a pneumatocyst (Fig. 8a). Cells lining the pneumatocyst secrete the gases or may expel out of it through one or more small openings called stigmata.
Thus pneumatophore is a balloon like structure or a hydrostatic chamber containing air.
When pneumatophore is filled with air, the colony becomes lighter and floats at the surface of the water, but when the gas is expelled out of the pneumatophore, colony sinks down.
b. Modifications of medusae . The medusoid individuals are of the following types
1. Nectophore or nectocalyx or swimming zooid with a muscular bell without manubrium or tentacles.
2. Pneumatophore or float as a bladder-like medusa filled with secreted gas.
3. Phyllozooid or bract, usually leaf-like and studded with nematocysts, serving for protection of other zooids.
4. Gonophore bearing gonads, which may be either male, producing sperms, or female producing ova.
Origin of polymorphism
As we have seen, colonies of Siphonophora represent the most specialized of Hydrozoa attaining the highest degree of polymorphism and presenting the greatest number of medusoid and polyploid types.
There are two views regarding which came first, polyp or medusa, during the evolution of polymorphism in Coelenterata.
According to one view, the ancestral coelenterate was a hydra-like polyp which arose from gastraea. It gave rise to hydroid colony by asexual budding. In the sessile colony some polyps became modified into medusae for sexual reproduction and pelagic life.
Thus, through division of labour, the hydroid colony became polymorphic.
According to second view (Brooks, 1886), which seems to be more acceptable, the ancestral coelenterata was a primitive medusa.
It arose from metagastreae by developing tentacles and becoming free swimming.
According to Huxley, and Metschnikoff, the manubrium, tentacles and umbrella of this p0rimitive medusoid individual were multiplied and shifted from their original positions to become various zooids of the polymorphic colony.
According to this view, polyploid stage is considered the persistent larval stage and medusoid the completely evolved coelenterate.
CORALS AND CORAL REEF FORMATION
Meaning of corals: Coral animals or corals are marine, mostly colonial, polyploid coelenterates, looking like miniature sea anemones and living in a secreted skeleton of their own.
Their calcareous or horny skeleton is also commonly known as coral.
Some corals grow into massive, solid structures others form large, branched colonies.
Most of the corals belongs to the class Anthozoa and a few to the class Hydrozoa of phylum coelenterata.
Structure of coral polyp:
1. soft structure
A typical coral polyp from a colony is a small organism about 10mm long and 1to 3 mm in diameter. Solitary coral polyps are much larger reaching up to 25 cm in diameter.
A basal disc is absent because the basal region of polyp is surrounded by calcareous exoskeleton.
Oral disc bears numerous tentacles, in several rows around an elongated, oval or circular mouth. Pharynx or stomodaeum is short and without siphonoglyphs.
Mesenteries are restricted to the upper part of coelenteron and mesenterial filaments contain only one glandular lobe bearing nematocysts.
A typical coral polyp from a colony is a small organism about 10mm long and 1to 3 mm in diameter. Solitary coral polyps are much larger reaching up to 25 cm in diameter.
A basal disc is absent because the basal region of polyp is surrounded by calcareous exoskeleton.
Oral disc bears numerous tentacles, in several rows around an elongated, oval or circular mouth. Pharynx or stomodaeum is short and without siphonoglyphs.
Mesenteries are restricted to the upper part of coelenteron and mesenterial filaments contain only one glandular lobe bearing nematocysts.
Living polyps are found only on surface layers of coral masses. They feed at night both by raptorial and suspension feeding. When not feeding they withdraw into cup-like cavities of skeleton.
Structure of coral skeleton:
Skeleton of a solitary coral is known as corallite. It is a calcareous exoskeleton secreted by epidermis.
In a colonial coral corallites of individual polyps fuse together to form a skeletal mass, called corallum.
Each corallite is like a stony cup with a basal part or basal plate, and a cup wall or theca, enclosing the aboral portion of the polyp.
Cavity of cup contain a number of vertical radiating ridges called sclerosepta, proceeding from theca towards the centre of the cup.
Inner ends of sclerosepta are fused to form an irregular central skeletal mass or columella.
Formation of coral skeleton:
In coral polyps sexual reproduction takes place by fusion of gametes. Zygote develops into a free swimming ciliated planula larva which settles down and metamorphosis into a young coral polyp.
There is no medusa stage. By asexual budding, single become s the parent of all other members of the colony.
The coral polyp begins to secrete a skeleton rudiment or prototheca. It is secreted by ectoderm, first as a basal plate. Following it, radial folds develop which secrete sclerosepta. At the same time a rim is built up as a thecal wall around the polyp, lying at the top.
Mean while further skeletol material is added into the gaps between sclerosepta of skeleton which usually alternates with mesenteries of the polyp.
Coral colony grows in size continuously by budding of new polyps, particularly along the margins and on surface layers of coral masses. Variety in form of compound corals results due to cavities patterns of budding.
Coral reefs
Meaning of coral reefs:
Coral colonies grow continuously in size by budding of polyps and often forms extensive masses, known as coral reefs.
A coral reef is a ridge or mound of limestone, the upper surface of which near the surface of sea and which is formed chiefly of CaCo3 secreted by coral polyps.
Principal builders of coral reefs are stony corals (Madreporaria), but other important contributors are the hydrocorallines and alcyonarians. Coralline algae and Foraminiferan Protozoa also take part in the formation of coral reefs.
Reef building corals require warm shallow waters (normally above 200C). They are therefore limited to the Indo-Pacific, the Central Western Pacific, and the Caribbean regions north of Bermuda.
Kinds of coral reefs:
There are three types of coral reefs:
• a. Fringing reefs
• b. Barrier reefs
• c. Atoll
a. Fringing reefs: Fringing reefs are developed in shallow waters on or near the shores of the volcanic islands. These are the simplest kind of reefs. They extend from the sea shore towards the sea as a platform ranging from few meters to half a kilometer and then slopes down towards the bottom of the sea
Fringing reefs consist of several zones that are characterized by their depth, the structure of the reef, and its plant and animal communities.
Reef edge or reef front
Seaward slope
Reef flat
Boulder zone
Lagoon
b. Barrier reefs.
A volcanic island with the fringing reefs is surrounded with a big channel of water called lagoon. Its depth may vary from 20 to 100 meters and even ships can pass through it. Lagoon is further surrounded by reefs called barrier reefs.
As their name suggests, they act as a barrier for ships between sea shore and the main sea.
Lagoon has fringing reefs towards the volcanic coast and barrier reefs on the other side of it. Sometimes both fringing reefs and barrier reefs may join each other at the bottom of the sea.
There is a Great Barrier Reefs of Australia. It is not a single structure but is made up of many strings of separate reefs joined to each other at the bottom and thus forms a very big structure which extends along the north eastern coast of Australia for over 2000 kilometers.
It is separated from the main land by a lagoon which is around 15 to 250 kilometers wide and 70 meters deep. During high tide, big ships can sail over it without realizing that reefs are present nearby and may crash. The Great Barrier Reef is the contribution of all different kinds of coral growth over the years.
c. Atoll: Atolls are coral reefs which are present within sea water hundreds or thousands of kilometers away from the nearest sea shore.
There is no volcanic island present. It is more or less circular or horse shoe shaped reef enclosing a central lagoon which may be 40 or 50 miles across and 20 to 90 meters deep.
It may be a complete or broken into many small reef islands separated from each other by water channels.
At some places reef is low so sea water simply covers it and reef is not visible. Sometimes a large atoll is formed by many small islets joined together along a line of reef. Thousands of such atolls are found in the South Pacific.
It must be noted that reefs are not continuous rigid structures but they are broken up into many reefs and islands by water channels. Suvadiva is the largest atoll present in Maldives.
Bikini Atoll has 2.87square miles land area with a lagoon area of 280 square miles. It was inhabited by the people but people moved to different places as it was selected by the United States for testing hydrogen and atomic bombs. Horse shoe shaped atoll of West Texas is 70 to 90 miles across and 1,000 meters thick.
Theories explaining the formation of coral reefs:
a. Darwin’s subsidence theory: Darwin believed that the reef began as fringing reefs on a sloping shore. Then the island subsides very slowly, so slowly that the reef grows upward at about the same rate, naturally the island becomes small, the channel between the reef and land widens and thus the fringing reef transforms into a barrier reef. Further subsidence of the land till it sinks completely out of site results in the formation of an atoll (Fig. 18). This is substantiated by the fact that all the known coral reefs were in regions where a sinking of the land was known to have taken place or where there were evidences that it had probably occurred.
b. Samper Murray solution theory: This theory states that calcareous skeletons of animals and other sediments form big mounds on the floor of the oceans. Over the time, these deposits grow to certain heights and corals grow on them and reach the water surface. Barrier reef is formed by the luxurious growth of coral at the outer edge while atoll is formed by dissolution of the inner coral rock.
c. Submerged bank theory This theory states that both barrier reefs and atolls grew upon pre-existing flat surfaces. Extensive coral growth occurred on a flat surface which got submerged in the water with the passage of time. Exposed regions formed the barrier reefs, while the shape of the atolls is obtained by the action of prevailing water currents and winds.
Significance of corals:
i. They show a high degree of physiological integration, division of labor, and perfect coordination with other groups of animals staying together, mutually benefiting each other in getting shelter, protection and food. Thus they constitute an ecologically important aquatic ecosystem.
ii. They help in studying the evolution of the animals as fossils of the animals are preserved in the coral reefs over the years.
iii. By studying the lines of growth on the fossils of some mollusk help in knowing about the seasonal fluctuations as the thickness of the growth line varies from season to season.
iv. A few stony corals because of the presence of minute pores are used in surgical procedures as human capillaries can easily pierce through equal sized pores which are helpful in interconnecting two bones with each other. These corals are being used in the surgery for bone grafts and jaw surgery etc.
v. Horseshoe shaped Atoll is the largest limestone reservoir for the oil production in North America.
vi. Many of the corals are precious stones which are used in making jewelry and have aesthetic value.
vii. Coral reefs are very hard structures and are an important source of mortar, cement, lime etc as they contain enough amount of CaCO3, therefore their rocks can be used for making roads and houses etc.
viii. Reefs are also a rich source for medical formulations, used to treat a wide range of diseases like asthma, heart diseases, and viral, fungal and bacterial infections. It has been reported in 2006 that Yellow coral (Isis hippuris) collected off the coast of Okinawa island of Japan has yielded a compound that can slow down and possibly prevent virus replication and also treat cancer.
ix. Lastly, corals act as affective buffers against erosion and storms occurring in the sea thus help in preventing tsunami disaster.
The presence of polymorphism in cnidarians is one of their characteristic features. It is defined as the occurrence of structurally and functionally different types of individuals within the same organism during its life cycle.
A species that exhibits this phenomenon is called polymorphic.
Polymorphism is predominantly exhibited by the different animals of class- hydrozoa.
Hydroid colonies which bear two types of zooids are known as dimorphic, while colonies which bear more than two types of zooids are called polymorphic colonies.
Two basic forms
In Hydrozoa (or Coelenterates), which may be single or colonial, there occur two main types of individuals or zooids- polyps and medusae.
1. Polyp: It is sessile (fixed to the substratum) with a hydra like body attached to the main colony by narrower end. Its free end is wider and raised into hypostome that bears a mouth surrounded by tentacles. It faces upwards and carries the function of feeding the colony. Polyps are specialized for feeding and thus known as gastrozooids.
2. Medusa: It is a free swimming zooid with an umbrella shaped body having exumbrellar and subumbrellar surfaces. It has a mouth facing downwards (present on the tubular growth called manubrium hanging down from subumbrellar surface) in contrast to polyp in which mouth faces upwards.
Like hydranth, it can feed on its own for its survival until its function is over. It bears four gonads (testis or ovary) which produces either sperms or ova at the time of maturity and is responsible for sexual reproduction therefore also known as gonozooid. They normally die after reproducing the colony. Therefore, they not only help in sexual reproduction but also play an important role in dispersal of the colony.
Importance of Polymorphism
Polymorphism is essentially a phenomenon of division of labour. Different functions are assigned to different individuals, rather than to parts or organs of one individual.
Thus, polyps are concerned with feeding, protection and asexual reproduction, while medusae are concerned with sexual reproduction.
Patterns of polymorphism
Degree of polymorphism varies greatly in different groups of Hydrozoa
1. Dimorphic. Simplest and commonest pattern of polymorphism is exhibited by many hydrozoan colonies like Obelia, Tubularia, Campanularia etc.
They have only two types of zooids. Gastrozoids or hydranths are concerned with feeding. While gonozoids or blastostyles with asexual budding forming sexual medusae or gonophores.
Such colonies, bearing only two types of individuals are called dimorphic and the phenomenon is termed dimorphism.
2. Trimorphic. Some forms like Plumularia, are trimorphic. Besides Gastrozooids and gonozooids, they also possess a third type of individuals, the dactylozooids. These are functionally non-feeding and defensive polyps bearing batteries of nematocysts.
3. Polymorphic. Coelenterates having more than three types of individuals are called polymorphic.
A somewhat greater degree of polymorphism is found in the encrusting colony of Hydractinia with five types of polyps each performing a specialized function.
These are: 1. Gastrozooids for feeding
2. Spiral dactylozooids for protection
3. long sensory tentaculozooids with sensory cells
4. skeletozooids as spiny projections of chitin
5. gonozooids or reproductive individuals, bearing male or female gonophores or medusae for sexual reproduction
Extreme examples of polymorphism are seen in the pelagic or swimming colonies of the orders
Siphonophora (Diphes, Halistemmia, Stephalia, physalia) and Chondrophora (porpita, velella).
Polymorphis reaches its peak in siphonophora.
(a) Modifications of polyps
Polyps structurally get modified into different types of zooids according to the requirement of an individual, which are described below:
i. Gastrozooids: These are feeding zooids and resemble the structure of polyp without usual tentacles. They are tubular, elongated, with a mouth facing towards the bottom of the colony.
ii. Dactylozooids: These are protective zooids. They may be also called as feelers or palpons. Structurally, they look like gastrozooids but are blind structures without any mouth.
iii. Gonozooids: They may resemble gastrozooids having mouth but are without tentacles and bear medusa. In other the gonozooids may form stalked branches bearing grape like structures called gonophores. Sometimes tentacles like dactylozooids are attached to them which are called gonopalpons.
iv.Pneumatophore:
It is a hydrostatic apparatus present in siphonophores. It is gas filled chamber that appear to be a highly modified polyps (although previously considered as derived from medusae).
It helps in keeping the body in an upright condition while floating.
It is without mesogloea but the umbrella cavity contains an air chamber called a pneumatocyst (Fig. 8a). Cells lining the pneumatocyst secrete the gases or may expel out of it through one or more small openings called stigmata.
Thus pneumatophore is a balloon like structure or a hydrostatic chamber containing air.
When pneumatophore is filled with air, the colony becomes lighter and floats at the surface of the water, but when the gas is expelled out of the pneumatophore, colony sinks down.
b. Modifications of medusae . The medusoid individuals are of the following types
1. Nectophore or nectocalyx or swimming zooid with a muscular bell without manubrium or tentacles.
2. Pneumatophore or float as a bladder-like medusa filled with secreted gas.
3. Phyllozooid or bract, usually leaf-like and studded with nematocysts, serving for protection of other zooids.
4. Gonophore bearing gonads, which may be either male, producing sperms, or female producing ova.
Origin of polymorphism
As we have seen, colonies of Siphonophora represent the most specialized of Hydrozoa attaining the highest degree of polymorphism and presenting the greatest number of medusoid and polyploid types.
There are two views regarding which came first, polyp or medusa, during the evolution of polymorphism in Coelenterata.
According to one view, the ancestral coelenterate was a hydra-like polyp which arose from gastraea. It gave rise to hydroid colony by asexual budding. In the sessile colony some polyps became modified into medusae for sexual reproduction and pelagic life.
Thus, through division of labour, the hydroid colony became polymorphic.
According to second view (Brooks, 1886), which seems to be more acceptable, the ancestral coelenterata was a primitive medusa.
It arose from metagastreae by developing tentacles and becoming free swimming.
According to Huxley, and Metschnikoff, the manubrium, tentacles and umbrella of this p0rimitive medusoid individual were multiplied and shifted from their original positions to become various zooids of the polymorphic colony.
According to this view, polyploid stage is considered the persistent larval stage and medusoid the completely evolved coelenterate.
CORALS AND CORAL REEF FORMATION
Meaning of corals: Coral animals or corals are marine, mostly colonial, polyploid coelenterates, looking like miniature sea anemones and living in a secreted skeleton of their own.
Their calcareous or horny skeleton is also commonly known as coral.
Some corals grow into massive, solid structures others form large, branched colonies.
Most of the corals belongs to the class Anthozoa and a few to the class Hydrozoa of phylum coelenterata.
Structure of coral polyp:
1. soft structure
A typical coral polyp from a colony is a small organism about 10mm long and 1to 3 mm in diameter. Solitary coral polyps are much larger reaching up to 25 cm in diameter.
A basal disc is absent because the basal region of polyp is surrounded by calcareous exoskeleton.
Oral disc bears numerous tentacles, in several rows around an elongated, oval or circular mouth. Pharynx or stomodaeum is short and without siphonoglyphs.
Mesenteries are restricted to the upper part of coelenteron and mesenterial filaments contain only one glandular lobe bearing nematocysts.
A typical coral polyp from a colony is a small organism about 10mm long and 1to 3 mm in diameter. Solitary coral polyps are much larger reaching up to 25 cm in diameter.
A basal disc is absent because the basal region of polyp is surrounded by calcareous exoskeleton.
Oral disc bears numerous tentacles, in several rows around an elongated, oval or circular mouth. Pharynx or stomodaeum is short and without siphonoglyphs.
Mesenteries are restricted to the upper part of coelenteron and mesenterial filaments contain only one glandular lobe bearing nematocysts.
Living polyps are found only on surface layers of coral masses. They feed at night both by raptorial and suspension feeding. When not feeding they withdraw into cup-like cavities of skeleton.
Structure of coral skeleton:
Skeleton of a solitary coral is known as corallite. It is a calcareous exoskeleton secreted by epidermis.
In a colonial coral corallites of individual polyps fuse together to form a skeletal mass, called corallum.
Each corallite is like a stony cup with a basal part or basal plate, and a cup wall or theca, enclosing the aboral portion of the polyp.
Cavity of cup contain a number of vertical radiating ridges called sclerosepta, proceeding from theca towards the centre of the cup.
Inner ends of sclerosepta are fused to form an irregular central skeletal mass or columella.
Formation of coral skeleton:
In coral polyps sexual reproduction takes place by fusion of gametes. Zygote develops into a free swimming ciliated planula larva which settles down and metamorphosis into a young coral polyp.
There is no medusa stage. By asexual budding, single become s the parent of all other members of the colony.
The coral polyp begins to secrete a skeleton rudiment or prototheca. It is secreted by ectoderm, first as a basal plate. Following it, radial folds develop which secrete sclerosepta. At the same time a rim is built up as a thecal wall around the polyp, lying at the top.
Mean while further skeletol material is added into the gaps between sclerosepta of skeleton which usually alternates with mesenteries of the polyp.
Coral colony grows in size continuously by budding of new polyps, particularly along the margins and on surface layers of coral masses. Variety in form of compound corals results due to cavities patterns of budding.
Coral reefs
Meaning of coral reefs:
Coral colonies grow continuously in size by budding of polyps and often forms extensive masses, known as coral reefs.
A coral reef is a ridge or mound of limestone, the upper surface of which near the surface of sea and which is formed chiefly of CaCo3 secreted by coral polyps.
Principal builders of coral reefs are stony corals (Madreporaria), but other important contributors are the hydrocorallines and alcyonarians. Coralline algae and Foraminiferan Protozoa also take part in the formation of coral reefs.
Reef building corals require warm shallow waters (normally above 200C). They are therefore limited to the Indo-Pacific, the Central Western Pacific, and the Caribbean regions north of Bermuda.
Kinds of coral reefs:
There are three types of coral reefs:
• a. Fringing reefs
• b. Barrier reefs
• c. Atoll
a. Fringing reefs: Fringing reefs are developed in shallow waters on or near the shores of the volcanic islands. These are the simplest kind of reefs. They extend from the sea shore towards the sea as a platform ranging from few meters to half a kilometer and then slopes down towards the bottom of the sea
Fringing reefs consist of several zones that are characterized by their depth, the structure of the reef, and its plant and animal communities.
Reef edge or reef front
Seaward slope
Reef flat
Boulder zone
Lagoon
b. Barrier reefs.
A volcanic island with the fringing reefs is surrounded with a big channel of water called lagoon. Its depth may vary from 20 to 100 meters and even ships can pass through it. Lagoon is further surrounded by reefs called barrier reefs.
As their name suggests, they act as a barrier for ships between sea shore and the main sea.
Lagoon has fringing reefs towards the volcanic coast and barrier reefs on the other side of it. Sometimes both fringing reefs and barrier reefs may join each other at the bottom of the sea.
There is a Great Barrier Reefs of Australia. It is not a single structure but is made up of many strings of separate reefs joined to each other at the bottom and thus forms a very big structure which extends along the north eastern coast of Australia for over 2000 kilometers.
It is separated from the main land by a lagoon which is around 15 to 250 kilometers wide and 70 meters deep. During high tide, big ships can sail over it without realizing that reefs are present nearby and may crash. The Great Barrier Reef is the contribution of all different kinds of coral growth over the years.
c. Atoll: Atolls are coral reefs which are present within sea water hundreds or thousands of kilometers away from the nearest sea shore.
There is no volcanic island present. It is more or less circular or horse shoe shaped reef enclosing a central lagoon which may be 40 or 50 miles across and 20 to 90 meters deep.
It may be a complete or broken into many small reef islands separated from each other by water channels.
At some places reef is low so sea water simply covers it and reef is not visible. Sometimes a large atoll is formed by many small islets joined together along a line of reef. Thousands of such atolls are found in the South Pacific.
It must be noted that reefs are not continuous rigid structures but they are broken up into many reefs and islands by water channels. Suvadiva is the largest atoll present in Maldives.
Bikini Atoll has 2.87square miles land area with a lagoon area of 280 square miles. It was inhabited by the people but people moved to different places as it was selected by the United States for testing hydrogen and atomic bombs. Horse shoe shaped atoll of West Texas is 70 to 90 miles across and 1,000 meters thick.
Theories explaining the formation of coral reefs:
a. Darwin’s subsidence theory: Darwin believed that the reef began as fringing reefs on a sloping shore. Then the island subsides very slowly, so slowly that the reef grows upward at about the same rate, naturally the island becomes small, the channel between the reef and land widens and thus the fringing reef transforms into a barrier reef. Further subsidence of the land till it sinks completely out of site results in the formation of an atoll (Fig. 18). This is substantiated by the fact that all the known coral reefs were in regions where a sinking of the land was known to have taken place or where there were evidences that it had probably occurred.
b. Samper Murray solution theory: This theory states that calcareous skeletons of animals and other sediments form big mounds on the floor of the oceans. Over the time, these deposits grow to certain heights and corals grow on them and reach the water surface. Barrier reef is formed by the luxurious growth of coral at the outer edge while atoll is formed by dissolution of the inner coral rock.
c. Submerged bank theory This theory states that both barrier reefs and atolls grew upon pre-existing flat surfaces. Extensive coral growth occurred on a flat surface which got submerged in the water with the passage of time. Exposed regions formed the barrier reefs, while the shape of the atolls is obtained by the action of prevailing water currents and winds.
Significance of corals:
i. They show a high degree of physiological integration, division of labor, and perfect coordination with other groups of animals staying together, mutually benefiting each other in getting shelter, protection and food. Thus they constitute an ecologically important aquatic ecosystem.
ii. They help in studying the evolution of the animals as fossils of the animals are preserved in the coral reefs over the years.
iii. By studying the lines of growth on the fossils of some mollusk help in knowing about the seasonal fluctuations as the thickness of the growth line varies from season to season.
iv. A few stony corals because of the presence of minute pores are used in surgical procedures as human capillaries can easily pierce through equal sized pores which are helpful in interconnecting two bones with each other. These corals are being used in the surgery for bone grafts and jaw surgery etc.
v. Horseshoe shaped Atoll is the largest limestone reservoir for the oil production in North America.
vi. Many of the corals are precious stones which are used in making jewelry and have aesthetic value.
vii. Coral reefs are very hard structures and are an important source of mortar, cement, lime etc as they contain enough amount of CaCO3, therefore their rocks can be used for making roads and houses etc.
viii. Reefs are also a rich source for medical formulations, used to treat a wide range of diseases like asthma, heart diseases, and viral, fungal and bacterial infections. It has been reported in 2006 that Yellow coral (Isis hippuris) collected off the coast of Okinawa island of Japan has yielded a compound that can slow down and possibly prevent virus replication and also treat cancer.
ix. Lastly, corals act as affective buffers against erosion and storms occurring in the sea thus help in preventing tsunami disaster.
Wednesday, July 5, 2017
SCOLIODON TYPE STUDY
Scoliodon
1. Distribution: The genus Scoliodon is widely distributed in the Indian, Pacific West Indies and eastern coasts of South America and Atlantic Oceans. Genus scoliodon is distinguished from other sharks, in having elongated snout, depressed head and compressed body.
2. The common Indian dog fish is Scoliodon sorrakowah which means ‘black shark’ in Tamil.
3. Habit and Habitat of Scoliodon: The shark is a marine, carnivorous and predaceous animal. It eats small pelagic schooling and bottom living bony fishes, including anchovies, codlet as well
as shrimps and cuttle fish.
4. External Structures of Scoliodon: Scoliodon is an elongated spindle shaped animal. It has a laterally compressed body. A fully developed specimen of the genus attains a length of about 60 cm.
5. The body is divisible into head, trunk and tail. The head is dorso-ventrally flattened and terminates anteriorly into a dorso-ventrally compressed snout. The dorsal side of Scoliodon is darkgrey while the underside is pale white.
6. The trunk is more or less oval in transverse section. It attains maximum thickness in the middle region and the body gradually tapers posteriorly into a long tail. The tail is also oval in crosssection and bears a heterocercal type of caudal fin, i.e., the posterior end of the vertebral col umn is bent upwards and lies in the dorsal or epichordal lobe.
7. Body surface is rough due to backwardly projecting spines of placoid scales embedded in the skin.
8. Eyes: Two prominent circular eyes are present. Each eye is provided with movable upper and lower eyelids. The third eyelid or nictitating membrane can cover the whole eye in emergency. The pupil is a vertical slit like aperture.
9. Body apertures: The following important apertures are present on the body surface
10. Mouth: The mouth is a very wide crescentic aperture lying on the ventral side of the head near its anterior end. It is bounded by upper and lower jaws; each is beset with one or two rows of sharply pointed and backwardly directed teeth to catch the slippery prey. The teeth are replaced if these are broken. The teeth of Scliodon are modified scales.
11. Nares: The nostrils are placed one at each angle of the mouth. These are exclusively olfactory in function and have no connection with the mouth cavity. Each nostril is partly covered by a small fold of skin.
12. External gill slits: Posterior to the eyes there are five vertical slits on each side. They are called gill or branchial slits. The branchial slits lead into the gill pouches which in turn open into the pharyngeal cavity.
13. Cloacal apertures. The cloaca opens to the exterior by a cloacal aperture which is located in between the two pelvic fins. The cloacal aperture is an elongated opening. The cloaca is a common chamber, into which anus, urinary and genital apertures open. On each side of the cloaca lies the abdominal pore.
14. Abdominal pores: The abdominal pores are paired structures and situated on elevated papillae to communicate the coelom to the outside. A faint lateral line is present. Beneath this line a canal is present. The canal opens to the exterior by minute pores at intervals. Many pores, called ampullary pores, are also present on the head.
15. Caudal pits. At the base of caudal fin, the tail bears two shallow depressions, one dorsal and one ventral, known as caudal pits, which are characteristic of genus Scoliodon.
16. Fins: As in other fishes, Scoliodon bears unpaired and paired fins which are actually flaplike integumentary extensions of the body. These are flexible and are stiffened by cartilaginous rods or horny fin rays. All the fins are directed backwards which is of positive advantage in swift forward movement in water.
17. Median unpaired fins: The fins under this category include two dorsal, one caudal and one ventral fin. The dorsal fins are triangular in outline. The anterior dorsal is larger and situated at about the middle of the body. The posterior dorsal is comparatively small and occupies a median position between the first dorsal and the tip of tail.
18. The caudal fin has one ill developed ventral lobe (hypochordal) which is divided into two parts. Two shallow depressions, called caudal pits, are regarded as the diagnostic features of the genus. These are pre sent at the root of the tail, one at the dorsal surface and another on the ventral. The median ventral fin is located in the midventral line and just anterior to the caudal fin.
19. Lateral paired fins: Two pectoral and two pelvic fins constitute the lateral paired fins. The pectoral fins are large and are situated posterior to the gill clefts. The pelvic fins are much smaller. In females, these are simple but in males each of them is connected with a copulatory organ called myxipterygium or clasper. Clasper is rod like in appearance having a dorsal groove leading to a siphon at its base.
Respiratory system
Since dogfish is an aquatic animal, it depends wholly upon oxygen dissolved in sea water for respiration. Thus, respiration is aquatic and carried on entirely by vascular gills.
I. Respiratory organs:
1. The respiratory organs of scoliodon consist of 5 pairs of gill pouches containing gills. Five gill pouches are present in a series on either side in the lateral wall of pharynx, behind the hyoid arch.
2. Each gill-pouch is compressed anterio-posteriorly. It opens into pharynx by a large internal brachial aperture and to outside by a narrow vertical external branchial aperture or gill-slit. Two adjacent gill-pouches are completely separated from each other by a vertical fibro-muscular partition, the interbranchial or gill septum.
3. The inner or pharyngeal border of each gill septum is supported by a cartilaginous visceral arch or gill arch with its slender branchial rays. The septum is covered by an epithelium and contains blood vessels, nerves, etc.
4. The mucous membranes of a septum is raised into numerous horizontal leaf-like folds, called gill lamellae or gill filaments.
5. These constitute the gill proper and are richly supplied with blood-capillaries. Each septum bears two sets of gill-lamellae, one on its anterior face and the other on its posterior face. Each set makes a half gill called demibranch or hemibranch, while both the sets attached to a gill arch and its gill septum constitute a complete gill called holobranch.
6. The posterior demibranch of a septum has longer lamellae than those of the anterior demibranch. A gill pouch thus contains two demibranchs belonging to two adjacent gills.
7. In Scoliodon, the hyoid arch bears only a demibranch on its posteror face, the first four branchial arches bear holobranchs, while the fifth branchial arch is a branch or without any gill.
Circulatory System of Scoliodon:
The circulatory system consists of:
(a) The circulatory fluid, called blood,
(b) The heart,
(c) The arteries and
(d) The veins.
Blood:
The blood consists of colourless plasma and corpuscles are suspended in the plasma. Two kinds of corpuscles are encountered; the RBC (or erythrocytes) and the WBC (or leucocytes). The erythrocytes are oval bodies containing a nucleus. The haemoglobin is present in the erythrocytes. The leucocytes are amoeboid in structure.
Heart:
• The heart is a bent muscular tube and consists of the receiving parts, comprising of a sinus venosus and a dorsally placed auricle, and the forwarding parts, consisting of a ventricle and a conus arteriosus.
• The heart is situated on the ventral side of the body between two series of gill pouches. Receiving parts of the heart: The sinus venosus is a thin walled tubular chamber. The sinus venosus is highly contractile and the beating of the heart originates from this part of the heart.
• Two great veins, the ductus Cuveiri, open into the sinus venosus, one on each lateral side. Two hepatic sinuses enter the sinus venosus posteriorly. The sinus venosus opens into the auricle by sinuauricular aperture which is guarded by a pair of valves.
• The auricle is a large, triangular and thin walled chamber situated dorsal to the ventricle but in front of the sinus venosus. The auricle communicates with the ventricle through a slit like auriculo ventricular aperture guarded by two lipped valves. The receiving chambers, (sinus venosus and auricle) receive the venous blood from all parts of the body.
Forwarding parts of the heart:
• The ventricle has a very thick muscular wall, the inner surface gives many muscular strands, thus giving it a spongy texture. It is an oval chamber and constitutes the most prominent part of the heart. The conus arteriosus is a stout median muscular tube arising from the ventricle. The lumen of the conus arteriosus is provided with two transverse rows of semilunar valves.
• To keep the valves in position the free ends of the valves are attached to the ventricular wall by fine tendinous threads, called chordae tendinae. The conus arteriosus is continued forward as the ventral aorta.
• The function of the heart is to receive the deoxygenated blood from all parts of the body and to pump it for aeration to the gills. Such a type of the heart is designated as the venous or branchial heart, because only the deoxygenated blood circulates through its different parts.
Arterial System of Scoliodon:
The arterial system of Scoliodon is divided into two distinct categories of arteries. These are:
(a) The afferent branchial arteries arising from the ventral aorta which bring the deoxygenated blood to gills for oxygenation and
(b) The efferent branchial arteries which originate from gills and convey the oxygenated blood to the different parts of the body.
Afferent branchial arteries:
• The ventral aorta is situated on the ventral surface of the pharynx and extends up to the posterior border or the hyoid arch. The ventral aorta divides into two branches called innominate arteries, which again bifurcates into the first and second afferent branchial arteries.
• The third, fourth and fifth afferent arteries arise from the ventral aorta. Each afferent branchial artery arises from the ventral aorta by independent opening except the anterior most pairs which arise by a common opening.
Efferent branchial arteries:
• The afferent branchial arteries break up into capillaries in the gills. From the gills the blood is collected by efferent branchial arteries. There are nine pairs of efferent branchial arteries and these are equally distributed on each side. The first eight arteries form a series of four complete loops around the first four gill slits and the ninth efferent branchial artery collects blood from the demibranch of the fifth gill pouch and from where blood is poured into the fourth loop.
• In addition to short longitudinal connectives connecting the four loops, these are further connected with each other by a network of longitudinal commissural vessels called the lateral hypobranchial chain.
• From each efferent branchial loop arises an epibranchial artery. The four pairs of epibranchials join in the middorsal line to form the dorsal aorta. The ninth efferent branchial artery has no epibranchial branch but joins with the eighth efferent branchial artery.
Anterior arteries:
• The head region gets the blood supply from the first efferent branchial artery and partly from the proximal end of the dorsal aorta. Arteries from the first efferent branchial (hyoidean efferent) are:
(a) The external carotid,
(b) The afferent spiracular and
(c) The hyodean epibranchial which in turn receives a branch from dorsal aorta.
• The external carotid artery originates from the first collector loop and divides into a ventral mandibular artery giving branches to the muscles of the lower jaw and a superficial hyoid artery which supplies the second ventral constrictor muscle, the skin and the subcutaneous tissue beneath the hyoid arch.
• The afferent spiracular artery after originating from the middle of the hyoidean efferent, proceeds for ward as the spiracular epibranchial artery and enters the cranial cavity. Just before its entry into the cranial cavity it sends a great ophthalmic artery to the eye ball.
• Immediately after the entry to the cranium it joins with a branch from the internal carotid to form the cerebral artery. The cerebral artery immediately divides into an anterior and a posterior cerebral artery which supply the brain. The hyoidean epibranchial artery runs forwards and inwards to the posterior border of the orbit and gets an anterior branch from the dorsal aorta.
It divides immediately into:
(a) the stapedial artery which gives off the inferior orbital artery and runs forward as the superior orbital artery supplying the six eye muscles and the superficial tissue above the auditory capsule. The superior orbital artery gives a large buccal artery which runs as the maxillonasal artery. The maxillonasal gives several arteries to the muscles of the upper jaw, the olfactory sac and the rostrum,
(b) the internal carotid artery passes inward and enters the cranium where it bifurcates into two branches. One of the branches unites with its fellow from the opposite side and other branch unites with the stapedial.
Dorsal aorta and its branches:
The dorsal aorta is formed by the union of epibranchial arteries. It runs posteriorly and is situated ven- tral to the vertebral column. It is continued up to the tip of the tail as the caudal artery. Along the anteroposterior direction the following arteries have their origin from the dorsal aorta:
(a) Several buccal and vertebral arteries are given off anteriorly.
(b) A pair of small subclavian arteries arises from near the origin of the fourth epibranchial arteries.
The subclavian artery gets the epicoracoid artery on its way and divides into:
(i) A branchial artery to the pectoral girdle and pectoral fin, (ii) An anterio lateral artery to the body musculature and (iii) A dorsolateral artery to the dorsal musculature.
(c) A large coeliacomesenteric artery arises just behind the origin of fourth epibranchial artery. It divides into a smaller coeliac artery and a larger anterior mesenteric artery. (d) A lienogastric artery originates posterior to the coeliaco mesenteric artery and gives off (i) An ovarian (in females) or spermatic artery (in males) to gonad, (ii) A posterior intestinal artery to the posterior part of the intestine, (iii) A posterior gastric to the posterior part of the cardiac stomach and (iv) A splenic artery to the spleen.
(e) Series of paired parietal arteries emerge out behind the subclavian artery. Each parietal gives a dorsal parietal artery and a ventral parietal artery. The dorsal parietal artery supplies the dorsolateral musculature, the vertebral column, the spinal cord and the dorsal fin. The ventral and the peritoneum. The ventral parietal parietal artery supplies the ventral muscles gives renal branches to the kidneys.
(f) A pair of iliac arteries extends to the pelvic fin as femoral arteries.
14. Venous System of Scoliodon:
The deoxygenated blood from the different parts of the body is returned to the heart by veins which form irregular blood sinuses throughout their courses. The existence of extensive blood sinuses is a charac teristic feature of the venous system of Scoliodon.
Scales in fishes
• In many vertebrates, the exoskeletal covering of body is made of two types of scales: epidermal and dermal. Epidermal scales are cornified derivatives of the Malpigian layer of epidermis. They are well developed in terrestrial vertebrates such as reptiles, birds and mammals.
• Dermal scales are mesenchymal in origin and especially developed in the fishes. They are small, thin cornified, calcareous or borny plates which fit closely together or overlap.
• As regards the arrangement of scales on piscine body is concerned, they are most often imbricated and thus, overlap like shingles on the roof, with their free margins directed towa4rds the tail, so as to minimize the friction of water. But sometimes total reversal of the pattern of arrangement is seen in some fishes.
• Among barbot (Lota) and freshwater eel (Anguilla) the pattern is mosaic rather than overlapping one another, they are separated minutely or meet their neighbours only at their margins. Scales vary in size and shape in different species.
• The body of all fishes except members of family Siluridae and a few bottom dwellers is covered by scales.
• Five types of dermal scales have been identified in fishes: cosmoid, placoid, ganoid, cycloid and ctenoid.
• 1. Cosmoid scales: These do not occur in living fishes. These were characteristic of certain ostracoderms, placoderms, and extinct sacropterygians (lobe finned fishes and lung fishes). These consisted of 4 distinct layers: an outermost thin enamel like ganoine, thick dentine like cosmine, spongy bone and innermost compact bone.
• 2. Placoid scales: These are characteristic of elasmobranc fishes only. Each placoid scale consists of a backwardly directed spine arising from a rounded or rhomboidal basal plate embedded in dermis.
• Spine is made of enamel like and basal plate of dentine like bony material. A pulp cavity inside spine opens through basal plate. Placoid scales are closely set together in skin giving it a sandpaper like quality.
3. Gonoid scales: Ganoid or rhomboid scales are thick, usually rhomboid or diamond-shaped plates closely fitted side by side, like tiles, providing a bony armour to the fish. In some cases they may overlap.
• Ganoid scales are characteristic of chondrosteans (Polypterus, Acipencer) and holosteans (Leipidosteus) so that these are often called ganoid fishes. Polypterus has palaeoniscoid ganoid scales composed of 3 layers: outer enamel like ganoine, middle dentine like cosmine and inner bony isopedine.
• Lepidosteus has lepidosteoid ganoid scales with only two layers: outer ganoine and inner isopedine.
4. Cycloid scales: Cycloid scales are thin flexible translucent plates, rather circular in outline, thicker in the centre and marked with several concentric lines of growth which cal be used for determining the age of the fish. They are composed of a thin upper layer of bone and a lower layer of fibrous connective tissue. They overlap each other, each scale embedded in a small pocket of dermis. Cycloid scales are found in lung fishes, surviving dipnoans some holosteans and the lower teleosteans such as carps, cods, etc.
5. Ctenoid scales: These are characteristic of modern higher teleosteans such as perch, sunfish, etc. in form, structure and arrangement they are similar to cycloid scales. They are more firmly attached and their exposed free hind parts which are not overlapped, bear numerous small comblike teeth or spines.
Intermediate types between cycloid and ctenoid scales also occur. Certain fishes, such as flounders, may bear both types, ctenoid scales dorsally and cycloid ventrally.
MIGRATION IN FISHES
1. Many fishes like birds perform seasonal migrations. The barracudas and swordfish of the warm (Xiphius gladus) of the warm tropical seas perform latitudinal migration, moving north in spring and south in autumn.
2. Some deep water fishes perform daily vertical migration. Migration is sometimes limited to freshwaters only; it is called potamodromy or limnodromy and sometimes limited within the sea called oceanodromy.
3. However, migration between the two is called diadromy, which is classified into two categories. Movement from fresh water to salt-water (sea) for spawning is called catadromous migration.
4. The most famous example of catadromous fish is the freshwater eel, Anguilla. The reverse movement, that is from salt-water to fresh, is termed anadromous migration. Examples of anadromous fishes are salmon, shad, striped bass, sturgeon, Alosa, Hilsa and some trout.
5. Completely free movement between fresh and marine water without the purpose of breeding is called amphidromous migration, exhibited by fishes like Megalopa, Chanos etc.
1. Eels: (a) The best example of catadromous migration is furnished by two common species of eels, Anguilla rostrata of European fresh water rivers and Anguilla vulgaris of America.
(b) With the advent of autumn, their colour changes from yellow to metallic silver. Feeding stops with the shrinking of their digestive tract.
(c) Eyes become larger, snout becomes sharper with thinner lips and the gonads are fully mature. The silvery eels then enter sea and migrate about 4500 kilomete4rs westwards from Europe or eastwards from America.
(d) Reaching their breeding place in the Sargasso sea off Bermuda, the adults die immediately after spawning in deep waters. The eggs hatch into little transparent, leaf-like flattened pelagic larvae, called leptocephalia, less than 6 mm long.
(e) They have sharp needle-like teeth for feeding. During their long return journey towards homes of their parents, they grow into elvers or glass eels about 8cm long, with cylindrical bodies.
(f) On reaching land, the males remain behind in brackish waters near coasts, while the females ascend freshwater streams and rivers. The elvers feed and grow to become yellow eels in some years. How the elvers without parents are able to find their way across the sea towards homes of their parents remains an unsolved mystery.
2. Salmon: (a) There is a single species of Atlantic salmon (Salmo salar) and 5 species of Pacific salmon (Oncorhynchus). They furnish the best example of anadromous migration.
(b) In winter, both the sexes leave their feeding grounds at sea to ascend the freshwater mountain streams, reaching the identical spot where they originally grew some years before. They stop feeding, change to a dull reddish brown from silver, and excavate shallow saucer-like pits in bottom gravel.
(c) After spawning the adults die, but some of the Atlantic species may survive, return to sea and spawn for a second or third time in life. After hatching the larval fish feed and grow for some time in the streams before going out to sea.
(d) The young salmons grow faster in the ocean because of abundant food there. Experimental evidence shows that strong olfactory sense of salmon determines its homing into the original birth place, for different streams have different odours.
1. Distribution: The genus Scoliodon is widely distributed in the Indian, Pacific West Indies and eastern coasts of South America and Atlantic Oceans. Genus scoliodon is distinguished from other sharks, in having elongated snout, depressed head and compressed body.
2. The common Indian dog fish is Scoliodon sorrakowah which means ‘black shark’ in Tamil.
3. Habit and Habitat of Scoliodon: The shark is a marine, carnivorous and predaceous animal. It eats small pelagic schooling and bottom living bony fishes, including anchovies, codlet as well
as shrimps and cuttle fish.
4. External Structures of Scoliodon: Scoliodon is an elongated spindle shaped animal. It has a laterally compressed body. A fully developed specimen of the genus attains a length of about 60 cm.
5. The body is divisible into head, trunk and tail. The head is dorso-ventrally flattened and terminates anteriorly into a dorso-ventrally compressed snout. The dorsal side of Scoliodon is darkgrey while the underside is pale white.
6. The trunk is more or less oval in transverse section. It attains maximum thickness in the middle region and the body gradually tapers posteriorly into a long tail. The tail is also oval in crosssection and bears a heterocercal type of caudal fin, i.e., the posterior end of the vertebral col umn is bent upwards and lies in the dorsal or epichordal lobe.
7. Body surface is rough due to backwardly projecting spines of placoid scales embedded in the skin.
8. Eyes: Two prominent circular eyes are present. Each eye is provided with movable upper and lower eyelids. The third eyelid or nictitating membrane can cover the whole eye in emergency. The pupil is a vertical slit like aperture.
9. Body apertures: The following important apertures are present on the body surface
10. Mouth: The mouth is a very wide crescentic aperture lying on the ventral side of the head near its anterior end. It is bounded by upper and lower jaws; each is beset with one or two rows of sharply pointed and backwardly directed teeth to catch the slippery prey. The teeth are replaced if these are broken. The teeth of Scliodon are modified scales.
11. Nares: The nostrils are placed one at each angle of the mouth. These are exclusively olfactory in function and have no connection with the mouth cavity. Each nostril is partly covered by a small fold of skin.
12. External gill slits: Posterior to the eyes there are five vertical slits on each side. They are called gill or branchial slits. The branchial slits lead into the gill pouches which in turn open into the pharyngeal cavity.
13. Cloacal apertures. The cloaca opens to the exterior by a cloacal aperture which is located in between the two pelvic fins. The cloacal aperture is an elongated opening. The cloaca is a common chamber, into which anus, urinary and genital apertures open. On each side of the cloaca lies the abdominal pore.
14. Abdominal pores: The abdominal pores are paired structures and situated on elevated papillae to communicate the coelom to the outside. A faint lateral line is present. Beneath this line a canal is present. The canal opens to the exterior by minute pores at intervals. Many pores, called ampullary pores, are also present on the head.
15. Caudal pits. At the base of caudal fin, the tail bears two shallow depressions, one dorsal and one ventral, known as caudal pits, which are characteristic of genus Scoliodon.
16. Fins: As in other fishes, Scoliodon bears unpaired and paired fins which are actually flaplike integumentary extensions of the body. These are flexible and are stiffened by cartilaginous rods or horny fin rays. All the fins are directed backwards which is of positive advantage in swift forward movement in water.
17. Median unpaired fins: The fins under this category include two dorsal, one caudal and one ventral fin. The dorsal fins are triangular in outline. The anterior dorsal is larger and situated at about the middle of the body. The posterior dorsal is comparatively small and occupies a median position between the first dorsal and the tip of tail.
18. The caudal fin has one ill developed ventral lobe (hypochordal) which is divided into two parts. Two shallow depressions, called caudal pits, are regarded as the diagnostic features of the genus. These are pre sent at the root of the tail, one at the dorsal surface and another on the ventral. The median ventral fin is located in the midventral line and just anterior to the caudal fin.
19. Lateral paired fins: Two pectoral and two pelvic fins constitute the lateral paired fins. The pectoral fins are large and are situated posterior to the gill clefts. The pelvic fins are much smaller. In females, these are simple but in males each of them is connected with a copulatory organ called myxipterygium or clasper. Clasper is rod like in appearance having a dorsal groove leading to a siphon at its base.
Respiratory system
Since dogfish is an aquatic animal, it depends wholly upon oxygen dissolved in sea water for respiration. Thus, respiration is aquatic and carried on entirely by vascular gills.
I. Respiratory organs:
1. The respiratory organs of scoliodon consist of 5 pairs of gill pouches containing gills. Five gill pouches are present in a series on either side in the lateral wall of pharynx, behind the hyoid arch.
2. Each gill-pouch is compressed anterio-posteriorly. It opens into pharynx by a large internal brachial aperture and to outside by a narrow vertical external branchial aperture or gill-slit. Two adjacent gill-pouches are completely separated from each other by a vertical fibro-muscular partition, the interbranchial or gill septum.
3. The inner or pharyngeal border of each gill septum is supported by a cartilaginous visceral arch or gill arch with its slender branchial rays. The septum is covered by an epithelium and contains blood vessels, nerves, etc.
4. The mucous membranes of a septum is raised into numerous horizontal leaf-like folds, called gill lamellae or gill filaments.
5. These constitute the gill proper and are richly supplied with blood-capillaries. Each septum bears two sets of gill-lamellae, one on its anterior face and the other on its posterior face. Each set makes a half gill called demibranch or hemibranch, while both the sets attached to a gill arch and its gill septum constitute a complete gill called holobranch.
6. The posterior demibranch of a septum has longer lamellae than those of the anterior demibranch. A gill pouch thus contains two demibranchs belonging to two adjacent gills.
7. In Scoliodon, the hyoid arch bears only a demibranch on its posteror face, the first four branchial arches bear holobranchs, while the fifth branchial arch is a branch or without any gill.
Circulatory System of Scoliodon:
The circulatory system consists of:
(a) The circulatory fluid, called blood,
(b) The heart,
(c) The arteries and
(d) The veins.
Blood:
The blood consists of colourless plasma and corpuscles are suspended in the plasma. Two kinds of corpuscles are encountered; the RBC (or erythrocytes) and the WBC (or leucocytes). The erythrocytes are oval bodies containing a nucleus. The haemoglobin is present in the erythrocytes. The leucocytes are amoeboid in structure.
Heart:
• The heart is a bent muscular tube and consists of the receiving parts, comprising of a sinus venosus and a dorsally placed auricle, and the forwarding parts, consisting of a ventricle and a conus arteriosus.
• The heart is situated on the ventral side of the body between two series of gill pouches. Receiving parts of the heart: The sinus venosus is a thin walled tubular chamber. The sinus venosus is highly contractile and the beating of the heart originates from this part of the heart.
• Two great veins, the ductus Cuveiri, open into the sinus venosus, one on each lateral side. Two hepatic sinuses enter the sinus venosus posteriorly. The sinus venosus opens into the auricle by sinuauricular aperture which is guarded by a pair of valves.
• The auricle is a large, triangular and thin walled chamber situated dorsal to the ventricle but in front of the sinus venosus. The auricle communicates with the ventricle through a slit like auriculo ventricular aperture guarded by two lipped valves. The receiving chambers, (sinus venosus and auricle) receive the venous blood from all parts of the body.
Forwarding parts of the heart:
• The ventricle has a very thick muscular wall, the inner surface gives many muscular strands, thus giving it a spongy texture. It is an oval chamber and constitutes the most prominent part of the heart. The conus arteriosus is a stout median muscular tube arising from the ventricle. The lumen of the conus arteriosus is provided with two transverse rows of semilunar valves.
• To keep the valves in position the free ends of the valves are attached to the ventricular wall by fine tendinous threads, called chordae tendinae. The conus arteriosus is continued forward as the ventral aorta.
• The function of the heart is to receive the deoxygenated blood from all parts of the body and to pump it for aeration to the gills. Such a type of the heart is designated as the venous or branchial heart, because only the deoxygenated blood circulates through its different parts.
Arterial System of Scoliodon:
The arterial system of Scoliodon is divided into two distinct categories of arteries. These are:
(a) The afferent branchial arteries arising from the ventral aorta which bring the deoxygenated blood to gills for oxygenation and
(b) The efferent branchial arteries which originate from gills and convey the oxygenated blood to the different parts of the body.
Afferent branchial arteries:
• The ventral aorta is situated on the ventral surface of the pharynx and extends up to the posterior border or the hyoid arch. The ventral aorta divides into two branches called innominate arteries, which again bifurcates into the first and second afferent branchial arteries.
• The third, fourth and fifth afferent arteries arise from the ventral aorta. Each afferent branchial artery arises from the ventral aorta by independent opening except the anterior most pairs which arise by a common opening.
Efferent branchial arteries:
• The afferent branchial arteries break up into capillaries in the gills. From the gills the blood is collected by efferent branchial arteries. There are nine pairs of efferent branchial arteries and these are equally distributed on each side. The first eight arteries form a series of four complete loops around the first four gill slits and the ninth efferent branchial artery collects blood from the demibranch of the fifth gill pouch and from where blood is poured into the fourth loop.
• In addition to short longitudinal connectives connecting the four loops, these are further connected with each other by a network of longitudinal commissural vessels called the lateral hypobranchial chain.
• From each efferent branchial loop arises an epibranchial artery. The four pairs of epibranchials join in the middorsal line to form the dorsal aorta. The ninth efferent branchial artery has no epibranchial branch but joins with the eighth efferent branchial artery.
Anterior arteries:
• The head region gets the blood supply from the first efferent branchial artery and partly from the proximal end of the dorsal aorta. Arteries from the first efferent branchial (hyoidean efferent) are:
(a) The external carotid,
(b) The afferent spiracular and
(c) The hyodean epibranchial which in turn receives a branch from dorsal aorta.
• The external carotid artery originates from the first collector loop and divides into a ventral mandibular artery giving branches to the muscles of the lower jaw and a superficial hyoid artery which supplies the second ventral constrictor muscle, the skin and the subcutaneous tissue beneath the hyoid arch.
• The afferent spiracular artery after originating from the middle of the hyoidean efferent, proceeds for ward as the spiracular epibranchial artery and enters the cranial cavity. Just before its entry into the cranial cavity it sends a great ophthalmic artery to the eye ball.
• Immediately after the entry to the cranium it joins with a branch from the internal carotid to form the cerebral artery. The cerebral artery immediately divides into an anterior and a posterior cerebral artery which supply the brain. The hyoidean epibranchial artery runs forwards and inwards to the posterior border of the orbit and gets an anterior branch from the dorsal aorta.
It divides immediately into:
(a) the stapedial artery which gives off the inferior orbital artery and runs forward as the superior orbital artery supplying the six eye muscles and the superficial tissue above the auditory capsule. The superior orbital artery gives a large buccal artery which runs as the maxillonasal artery. The maxillonasal gives several arteries to the muscles of the upper jaw, the olfactory sac and the rostrum,
(b) the internal carotid artery passes inward and enters the cranium where it bifurcates into two branches. One of the branches unites with its fellow from the opposite side and other branch unites with the stapedial.
Dorsal aorta and its branches:
The dorsal aorta is formed by the union of epibranchial arteries. It runs posteriorly and is situated ven- tral to the vertebral column. It is continued up to the tip of the tail as the caudal artery. Along the anteroposterior direction the following arteries have their origin from the dorsal aorta:
(a) Several buccal and vertebral arteries are given off anteriorly.
(b) A pair of small subclavian arteries arises from near the origin of the fourth epibranchial arteries.
The subclavian artery gets the epicoracoid artery on its way and divides into:
(i) A branchial artery to the pectoral girdle and pectoral fin, (ii) An anterio lateral artery to the body musculature and (iii) A dorsolateral artery to the dorsal musculature.
(c) A large coeliacomesenteric artery arises just behind the origin of fourth epibranchial artery. It divides into a smaller coeliac artery and a larger anterior mesenteric artery. (d) A lienogastric artery originates posterior to the coeliaco mesenteric artery and gives off (i) An ovarian (in females) or spermatic artery (in males) to gonad, (ii) A posterior intestinal artery to the posterior part of the intestine, (iii) A posterior gastric to the posterior part of the cardiac stomach and (iv) A splenic artery to the spleen.
(e) Series of paired parietal arteries emerge out behind the subclavian artery. Each parietal gives a dorsal parietal artery and a ventral parietal artery. The dorsal parietal artery supplies the dorsolateral musculature, the vertebral column, the spinal cord and the dorsal fin. The ventral and the peritoneum. The ventral parietal parietal artery supplies the ventral muscles gives renal branches to the kidneys.
(f) A pair of iliac arteries extends to the pelvic fin as femoral arteries.
14. Venous System of Scoliodon:
The deoxygenated blood from the different parts of the body is returned to the heart by veins which form irregular blood sinuses throughout their courses. The existence of extensive blood sinuses is a charac teristic feature of the venous system of Scoliodon.
Scales in fishes
• In many vertebrates, the exoskeletal covering of body is made of two types of scales: epidermal and dermal. Epidermal scales are cornified derivatives of the Malpigian layer of epidermis. They are well developed in terrestrial vertebrates such as reptiles, birds and mammals.
• Dermal scales are mesenchymal in origin and especially developed in the fishes. They are small, thin cornified, calcareous or borny plates which fit closely together or overlap.
• As regards the arrangement of scales on piscine body is concerned, they are most often imbricated and thus, overlap like shingles on the roof, with their free margins directed towa4rds the tail, so as to minimize the friction of water. But sometimes total reversal of the pattern of arrangement is seen in some fishes.
• Among barbot (Lota) and freshwater eel (Anguilla) the pattern is mosaic rather than overlapping one another, they are separated minutely or meet their neighbours only at their margins. Scales vary in size and shape in different species.
• The body of all fishes except members of family Siluridae and a few bottom dwellers is covered by scales.
• Five types of dermal scales have been identified in fishes: cosmoid, placoid, ganoid, cycloid and ctenoid.
• 1. Cosmoid scales: These do not occur in living fishes. These were characteristic of certain ostracoderms, placoderms, and extinct sacropterygians (lobe finned fishes and lung fishes). These consisted of 4 distinct layers: an outermost thin enamel like ganoine, thick dentine like cosmine, spongy bone and innermost compact bone.
• 2. Placoid scales: These are characteristic of elasmobranc fishes only. Each placoid scale consists of a backwardly directed spine arising from a rounded or rhomboidal basal plate embedded in dermis.
• Spine is made of enamel like and basal plate of dentine like bony material. A pulp cavity inside spine opens through basal plate. Placoid scales are closely set together in skin giving it a sandpaper like quality.
3. Gonoid scales: Ganoid or rhomboid scales are thick, usually rhomboid or diamond-shaped plates closely fitted side by side, like tiles, providing a bony armour to the fish. In some cases they may overlap.
• Ganoid scales are characteristic of chondrosteans (Polypterus, Acipencer) and holosteans (Leipidosteus) so that these are often called ganoid fishes. Polypterus has palaeoniscoid ganoid scales composed of 3 layers: outer enamel like ganoine, middle dentine like cosmine and inner bony isopedine.
• Lepidosteus has lepidosteoid ganoid scales with only two layers: outer ganoine and inner isopedine.
4. Cycloid scales: Cycloid scales are thin flexible translucent plates, rather circular in outline, thicker in the centre and marked with several concentric lines of growth which cal be used for determining the age of the fish. They are composed of a thin upper layer of bone and a lower layer of fibrous connective tissue. They overlap each other, each scale embedded in a small pocket of dermis. Cycloid scales are found in lung fishes, surviving dipnoans some holosteans and the lower teleosteans such as carps, cods, etc.
5. Ctenoid scales: These are characteristic of modern higher teleosteans such as perch, sunfish, etc. in form, structure and arrangement they are similar to cycloid scales. They are more firmly attached and their exposed free hind parts which are not overlapped, bear numerous small comblike teeth or spines.
Intermediate types between cycloid and ctenoid scales also occur. Certain fishes, such as flounders, may bear both types, ctenoid scales dorsally and cycloid ventrally.
MIGRATION IN FISHES
1. Many fishes like birds perform seasonal migrations. The barracudas and swordfish of the warm (Xiphius gladus) of the warm tropical seas perform latitudinal migration, moving north in spring and south in autumn.
2. Some deep water fishes perform daily vertical migration. Migration is sometimes limited to freshwaters only; it is called potamodromy or limnodromy and sometimes limited within the sea called oceanodromy.
3. However, migration between the two is called diadromy, which is classified into two categories. Movement from fresh water to salt-water (sea) for spawning is called catadromous migration.
4. The most famous example of catadromous fish is the freshwater eel, Anguilla. The reverse movement, that is from salt-water to fresh, is termed anadromous migration. Examples of anadromous fishes are salmon, shad, striped bass, sturgeon, Alosa, Hilsa and some trout.
5. Completely free movement between fresh and marine water without the purpose of breeding is called amphidromous migration, exhibited by fishes like Megalopa, Chanos etc.
1. Eels: (a) The best example of catadromous migration is furnished by two common species of eels, Anguilla rostrata of European fresh water rivers and Anguilla vulgaris of America.
(b) With the advent of autumn, their colour changes from yellow to metallic silver. Feeding stops with the shrinking of their digestive tract.
(c) Eyes become larger, snout becomes sharper with thinner lips and the gonads are fully mature. The silvery eels then enter sea and migrate about 4500 kilomete4rs westwards from Europe or eastwards from America.
(d) Reaching their breeding place in the Sargasso sea off Bermuda, the adults die immediately after spawning in deep waters. The eggs hatch into little transparent, leaf-like flattened pelagic larvae, called leptocephalia, less than 6 mm long.
(e) They have sharp needle-like teeth for feeding. During their long return journey towards homes of their parents, they grow into elvers or glass eels about 8cm long, with cylindrical bodies.
(f) On reaching land, the males remain behind in brackish waters near coasts, while the females ascend freshwater streams and rivers. The elvers feed and grow to become yellow eels in some years. How the elvers without parents are able to find their way across the sea towards homes of their parents remains an unsolved mystery.
2. Salmon: (a) There is a single species of Atlantic salmon (Salmo salar) and 5 species of Pacific salmon (Oncorhynchus). They furnish the best example of anadromous migration.
(b) In winter, both the sexes leave their feeding grounds at sea to ascend the freshwater mountain streams, reaching the identical spot where they originally grew some years before. They stop feeding, change to a dull reddish brown from silver, and excavate shallow saucer-like pits in bottom gravel.
(c) After spawning the adults die, but some of the Atlantic species may survive, return to sea and spawn for a second or third time in life. After hatching the larval fish feed and grow for some time in the streams before going out to sea.
(d) The young salmons grow faster in the ocean because of abundant food there. Experimental evidence shows that strong olfactory sense of salmon determines its homing into the original birth place, for different streams have different odours.
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