HAEMOCOELOMIC SYSTEM OF LEECH

Haemocoelomic system of Leech
I) Haemocoelomic channels
II) Circulation of Haemocoelomic fluid
Four longitudinal sinuses or channels
A) One dorsal
B) One ventral and
C) Two lateral channels & their branches
 These channels are not true blood vessels but remnants of original coelom
 Dorsal and ventral channels- non-contractile thin walls made of connective tissue and coelomic epithelium
 Lateral channels- secondarily acquire contractile and thick muscular walls and appear like true blood vessels
 Through haemocoelomic channels- circulates the red-coloured haemocoelomic fluid containing amoeboid corpuscles and dissolved haemoglobin.
 Amoeboid corpuscles or coelomocytes-phagocytic in nature
A) Dorsal Channel
 Runs mid-dorsally, below body wall, attached to the alimentary canal
 Takes up a sinuous course
 Anteriorly runs up to 6th segment and breaks up into smaller branches and capillaries, which extend into the first five segments.
 Posteriorly, it bifurcates in 22nd segment
 The two branches of the bifurcation passes ventrally around rectum to join the posterior dilatation of ventral channel
 Has thin, non-contractile, wall and is without valves
 Its haemocoelomic fluid runs from posterior to anterior side.
 It is a distributing channel
It gives out two types of branches
1) Dorso-laterals
2) Dorso-intestinals

1) Dorso-laterals
 Two pairs arise ventro-laterally from dorsal channel in each segment
 Each branch runs outwards to form a capillary plexus in dorsal and dorso-lateral regions of body wall
2) Dorso-intestinals
 These are numerous small branches arising mid-ventrally from dorsal channel, all along its length.
 These supply the gut wall
B) Ventral Channel
 Runs mid-ventrally beneath alimentary canal, from one end of body to other, along a straight course.
 Wider than dorsal channel and encloses the entire central nervous system including nerve ring and ventral nerve cord
 Somewhat dilated around the segmental nerve ganglia and the terminal ganglionic mass.
 Non-contractile and is without valves
 Acts as a distributing channel
 Haemocoelomic fluid flows from anterior to posterior side
 Gives out two pairs of branches in each segment
a) First pair:
 First pair or cutaneous branches are given out at the level of segmental nerve ganglion
 Branch of each side at once bifurcates into a ventral branch, forming a capillary network in the ventro-lateral region of body wall, and an abdomino-dorsal branch, which runs vertically upwards with the dorso-ventral muscle and forms dorso-lateral cutaneous plexus.
a) Second pair:
 second pair or nephridial branches are given out just behind the segmental ganglion of ventral nerve cord
 Nephridial branch of each side runs outwards, and reaches the testis sac of its side
 It widens into two or three closely set saccules, the perinephrostomial ampullae, containing a ciliated organ
 The branch then continues outwards to supply the bodywall, sending on its way, a small branch to the nephridium
 Ciliated organ manufactures coelomic corpuscles for the haemocoelomic system and its cilia help in the circulation of haemocoelomic fluid.
 There are only 11 pairs of nephridial branches, one pair in each of the segments 12 to 22, which contain testis sacs.
Lateral Channels
 There are two lateral haemocoelomic channels placed one on either side of alimentary canal
 Wide and uniform in diameter, with thick muscular and contractile walls
 Has valves like true blood vessels
 Haemocoelomic fluid flows in them from behind forwards.
 They are collecting as well as distributing channels
 In each segment, each lateral channel receives on its outer side two branches
a) latero-lateral
b) latero-dorsal and gives off on the inner side one branch, the
c) latero-ventral
a) latero-lateral:
 A short latero-lateral is formed by branches from lateral region of body wall and nephridia
 It joins the lateral channel at the level of nephridial vesicle.
b) latero-dorsal
 A large latero-dorsal is formed by branches from dorsal and dorso-lateral regions of body wall, gut wall and nephridium.
 Joins the lateral channel at the level of main lobe of nephridium
 Two latero-dorsals of opposite sides are connected by a transverse loop above the dorsal channel
 There are 17 such loops, called the dorsal commissures of the lateral channels, one in each segment from segments 6 to 22.
 Latero-dorsal is also connected with the latero-lateral of its own side by a longitudinal lateral commissure
 Latero-laterals and latero-dorsals are collecting branches and their openings into lateral channels are guarded by valves.

c) latero-ventral
 arises from the inner side of lateral channel and at once gives off a branch to supply nephridium and ventro-lateral regions of body wall.
 Then it bifurcates into two diverging branches, anterior and posterior. These unite with corresponding fellows of opposite side, beneath ventral channel, by the ventral commissures of the laterals, forming a characteristic rhomboidal figure.
 18 such rhomboids are present, one in each segment from 6th to 23rd segment.
 Rhomboids of adjacent segments are also connected together by three longitudinal intersegmental commissures, one median and two lateral.
 Latero-ventrals supply branches to nephridia, ventral side of alimentary canal and reproductive organs
 Anteriorly, both lateral channels break up in 6th segment into capillaries, while posteriorly they open into the dilatation of ventral channel where all the four channels are in direct communication.