NERVOUS SYSTEM OF PILA

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.