(i) Dorsal channel: It runs above alimentary canal, anteriorly upto 6th segment and then breaks up into smaller branches and capillaries, which extend into the first five segments, both the branches extend, surrounding the ventral channels and rectum to join the posterior dilation of ventral channel.
Dorsal channel has a thin non-contractile wall and is devoid of valves. It is a distributing channel and its haemocoelomic fluid runs from posterior to anterior side. It gives out two types of branches: (a) Dosolaterals: In each segment two pairs of dorsolateral channels arise from dorsal channel. They make capillary plexus and supply to the dorsal and dorso-lateral regions of body wall. (b) Dorso intestinals: All along its length dorsal channel gives rise to numerous dorso-intestinals to supply gut wall. (ii) Ventral channel: It runs beneath the alimentary canal from one end of the body to other along a straight course. It is wider and encloses the entire central nervous system including nerve ring and ventral nerve cord. Like dorsal channel it is also a distributing channel but in it haemocoelomic fluid runs from anterior to posterior side.
It gives out two pairs of branches in each segment. (a) Cutaneous branches: In each segment this branch again gives two branches, one ventral branch forming a capillary network in the ventrolateral row of body wall and the other abdomino-dorsal branch which runs vertically up in the body wall and forms dorso-lateral cutaneous plexus. (b) Nephridial branch: Nephridial branch of each side runs outward and reaching over the testis sac of its side, it widens into 2 or 3 closely set saccules, the perinephrostomial ampullae which contain a ciliated organ. This branch supplies the body wall and nephirdia. This branch arises only from the segments 12 to 22, which contain testis sac. Ciliated organ manufactures coelomic corpuscles for the haemocoelomic system and its cilia are believed to help in the circulation of haemocoelomic fluid. (iii) Lateral channels: They are like true blood vessels on account of having contractile walls and valves.
Haemocoelomic fluid flows in them from behind to forward. They are collecting as well as distributing channels. They collect the blood through latero-lateral and latero-dorsal channels while distribute it though latero-ventral channel.
(a) Latero-lateral: It collects the blood from the capillaries of lateral region of body wall and nephridia. It joins the lateral channel at the level of nephridial vesicle. (b) Latero-dorsal: This large channel is formed by branches from the dorsal and dorso-lateral region of body wall, gut wail and nephridium. Both the channels of each side are connected by a transverse loop above the dorsal channel. These are 17 in number from 6th to 22nd segments and are called dorsal commissures.
Latero-dorsal and latero-lateral channels are connected by lateral commissure. (c) Latero-ventral: It arises from the lateral channel and gives off at once a branch to supply nephridium and ventro-lateral regions of body wall. Then it gives two branches, anterior and posterior. These unite with their corresponding fellow of opposite side by ventral commissure.
In each segment, ventral commissures make 18 such rhomboids are seen from 6th to 23rd segment. Each rhomboid further communicates with each other by three intersegmental commissures. Latero-ventral channel supplies 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 dilation of ventral channel where all the four channels are in direct communication.