The basic unit of excretory system is nephridia, arranged segmentally from 6th to 22nd segment.
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(i):
From 6th to 11th segment the six pairs of nephridia are called pretesticular nephridia. Because testis sac is absent and initial lobe of the nephridia terminates in the loose connective tissue of body wall instead of testis sac. Rest of the structure of nephridia is same with testicular nephridia.
(ii) Testicualr nephridia:
A typical testicular nephridia is horse shoe shaped and consists of 6 parts
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(a) Main lobe:
It forms the major part of the nephridium and lies in a ventro-lateral position between two adjancent caeca of crop.
It has two limbs, one is longer and the other is shorter. Cells of main lobe are big and polyhedral in shape.
(b) Vesicle and the vasicle duct:
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From the end of longer anterior limb, a vesicle arises and runs posteriorly to open into a large bladder called terminal vesicle.
This is situated ventro-laterally behind the rest of nephridium. Vesicle is a large oval sac bounded with a non-contractile and thin wall.
It opens outside through the excretory duct by nephridiopore. At its origin from the vesicle, excretory duct is provided with a sphinctor muscle that controls the flow of excretory substances out of the vesicle.
(c) Apical lobe:
From the posterior limb of main lobe, arises a stout apical lobe. Its anterior end is slightly swollen and bent on itself like the handle of a walking stick. Its cells are big and of Leech traversed by regular intracellular canals.
(d) Initial lobe:
This lobe is a cord-like structure that is coiled around apical lobe. Its posterior end joins the main lobe while it’s anterior end runs inward up to the testis sac and blindly ends near the perinephrostomial ampulla. The intra cellular canal of initial lobe gives off many diverticula in each cell.
(e) Inner lobe:
It extends between the anterior and posterior limb of main lobe and runs along the outer side of apical lobe for about half of its length.
(f) Ciliated organ:
Ciliated organ is hanging from the inner wall of perinephrostomial ampulla with the help of 4 to 5 strands or trabeculae.
It is a modified and compound structure. It consists of a spongy central reservoir and ciliated funnels.
Outer wall of central mass is perforated and each pore encloses a funnel, while the inner connective tissue manufactures coelomic corpuscles.
In the embryo, ciliated organ has a distinct cellular connection with the nephridium. But in adult it loses its connection and excretory function and becomes a part of the haemocoelomic system by producing coelomic corpuscles.
Physiology of excretion:
Usually Hirudinaria is ammonotelic but secretes urea also in little quantity. Nephridium proper meant for excretion is profusely supplied with branches of haemocoelomic channels. Its gland cells separate waste products from haemcoelomic fluid.
Excretory fluid is finally collected into terminal vescicle and is discharged outside through nephridiopore.
Several workers have also attributed an excretory function to the botryoidal tissue, which communicates with the haemocoelomic vessels through its intra cellular capillaries.