EXCRETORY SYSTEM OF LEECH

Excretory system of Leech
Excretory system consists of 17 pairs of small coiled tubes, the nephridia, arranged segmentally, one pair in each segment from 6th to22nd.
Nephridia are of two types
i ) testicular
ii) pre-testicular
i) Testicular nephridia
 Posterior 11 pairs of nephridia lying one pair in each segment from 12 to 22nd, are termed testicular nephridia.
 They are called testicular nephridia, due to the presence of a pair of testis sacs in each of these segments.
 A typical testicular nephridium is a horseshoe-shaped structure traversed by a complicated system of canals
It consists of 6 parts:
1) main lobe
2) vesicle and vesicle duct
3) apical lobe
4) inner lobe
5) initial lobe
6) ciliated organ
1) Main lobe:
 Forms the horseshoe proper
 Lies in a ventro-lateral position between two adjacent caeca of crop
 Forms the major part of nephridium
 Consists of two unequal limbs
 One limb is longer and anterior in position and the other shorter and posterior
 Cells of main lobe are big and polyhedral in shape

2) Vesicle and the Vesicle duct.
 A narrow vesicle duct arises from the inner ventral end of anterior limb of main lobe
 Runs posteriorly to open into a large bladder or terminal vesicle, situated ventro-laterally behind the rest of nephridium
 Vesicle is a large oval sac, with a non-contractile thin wall, internally lined by a ciliated epithelium
 A short and narrow excretory duct, lined with a non-ciliated epithelium, leads from vesicle to open to the exterior through a rounded aperture, the nephridiopore
 At its origin from vesicle, the excretory duct is provided with a sphincter muscle that controls the flow of excretory substances out of the vesicle.
 M.L. Bhatia (1940)- bladder is lined by cilia
 B. Dev- the so called cilia are in fact non-motile bacteria, the nephridial microflora, 2.8 to 7 microns in length
3) Apical lobe
 Inner free end of posterior limb of main lobe is continued to form a stout apical lobe
 Present antero-posteriorly beneath the crop
 Its anterior end is slightly swollen and bent on itself like the handle of a walking stick
 Its cells are big and traversed by regular intracellular canals.
4) Inner lobe or incurrent lobe
 seen extending between the anterior and posterior limbs of main lobe
 also runs forward along the outer side of apical lobe for about half of its length
5) Initial lobe
 Long, narrow, transparent and cord-like structure
 Formed of a single row of elongated tubular cells and closely coiled around the apical lobe.
 Its posterior end joins the main lobe, while its anterior end runs inwards and reaches over the testis sac of its own side, where it ends blindly close to the perinephrostomial ampullae.
 The intracellular canal of initial lobe gives off many diverticula in each cell.
6) Ciliated organ
 Present inside peristomial ampullae
 Suspended from the inner walls of ampullae by 4 to 5 strands or trabeculae.
 It corresponds to the funnel or nephrostome of a typical annelid nephridium, but is a greatly modified and compound structure
 Ciliated organ consists of a spongy central reservoir and ciliated funnels
 The reservoir contains the central mass of connective tissue cells which manufacture the coelomic corpuscles
 Outer wall of central mass, made of a single layer of cells, and bears numerous minute pores.
 A ciliated funnel fits into each pore on the outside
 Each funnel is like an ear lobe, with about one-fourth of its margin incomplete
 Funnel covered with outwardly directed cilia on its outer margin and inner surface
 In the embryo, ciliated organ has a distinct cellular connection with the nephridium.
 But, in adult Hirudinaria, it loses the connection as well as excretory function and becomes a part of the haemocoelomic system.
 It manufactures coelomic corpuscles for the haemocoelomic system.
II Pre-testicular nephridia
 First six pairs of nephridia are termed pre-testicular nephridia because of their location in segments 6 to 11 without testis sacs themselves, but in front of those containing testis sacs.
 These nephridia resemble testicular nephridia in all respects except that their initial lobes end loosely in general connective tissue on their side of ventral nerve cord.
 There are no testis sacs, peri-nephrostomial ampullae and ciliated organs in their segments.