ANNELIDA- EXCERETORY SYSTEM OF LEECH

The excretory system of leech can be studied under the two headings-

1. Excretory organ
2. Process of excretion

Excretory organ-

Nephridia are the main excretory organ of leech. Nephridia are segmentally arranged from 6^th to 23^rd segment. Each has hollow, microscopic tubes arranged in pairs. There are 17 pairs nephridia are found in Leech.

These are of two types :-

1.  Pretesticular Nephridia - 6 pairs
2.  Testicular Nephridia- 11 Pairs

Pretesticular Nephridia :-

 This type of Nephridia are found from 6^th to 11^th segment. This type of nephridia has no connection with testis; hence called as pretesticular nephridia. Haemocoelomic ampullae and ciliated organs are absent in this type of nephridia. In other structures they are similar with testicular nephridia.

 Testicular Nephridia :-

Testicular Nephridia are found from 12^th to 22^nd segments. They are attached with testis sacs. This is called typical nephridia and 11 pairs in number. Each testicular nephridium is horse shoe shaped in outline. It consists of six parts-  

     1.Main lobe   2.Vesicle and vesicle duct   3.Apical lobe  4.Inner lobe   5.Initial lobe   6.Ciliated organ

Main lobe:-

It is exactly horse shoe proper. Main lobe of the nephridium lies between the caeca of two crops. It is the main parts of nephridium and divisible into.

• Long anterior part - Anterior limb
• Short posterior part - Posterior limb

The cells of main lobe, consists of glandular polyhedral cells. The cells are connected with small canaliculi. Canaliculi of main lobe opens into central canal.

Anterior limb opens into vesicle duct and posterior limb connected with apical lobe.

Vesicle and vesicle duct –

The anterior limb of the main lobe continuous to form a duct called vesicle duct. Vesicle duct bends posteriorly and opens into vesicle.

Vesicle is a large, oval, non-contractile, sac like structure. The wall is thin and inner lining is ciliated.

 A small thin tube arises from vesicle and opens outside ventrally through nephridiopore. Sphincter muscles are found around nephridiopore.

 Apical lobe –

Posterior limb of the main lobe proceeds forward and forms apical lobe. It is found below the crop. Apical lobe is thick, stout and bends. It appears like the head of walking stick. The cells of apical lobes are larger. Intracellular canals are found in it. This canal opens into central canal.

 Inner lobe –

 It is a short lobe called as incurrent lobe. It is found between anterior and posterior limb. Inner lobe is continued to the outer border of apical lobe up to middle. It joins with main lobe in posterior side.

Initial lobe –

It is associated with apical lobe. It is thin long transparent cord like structure. It surrounds the apical lobe. Initial lobe is formed of single row of hollow cells. The anterior end of this lobe is closed but the posterior end opens into main lobe. Anterior end is situated near perinephrostomial ampulla on testis.

Intra cellular canal is present in the initial lobe. Intra cellular canal gives rise to diverticula in each cell.

Ciliated organ –

Ciliated organs are peculiar structure, present in perinephrostomial ampulla. Ciliated organs are suspended from the wall of ampulla by 4-5 strands or trabeculae.

Ciliated organs are similar to the nephrostome of the typical nephridium. Ciliated organ is a modified compound structure, divides into two parts-  

1. Central reservoir – Spongy
2. Ciliated funnel

Reservoir- It is formed of connective tissue spongy central mass. It forms coelomic corpuscles. The wall of this is unicellular a perforated. This aperture opens into ciliated funnel.

Ciliated funnel – Each ciliated funnel is small earlobe like. Th margins are ciliated.

In embryonic stage, ciliated organs are attached with nephridium but in adult Hirudinaria (Leech), it looses its connection. Ciliated organs are non-functional in adult. It becomes a part of haemocoelomic system and forms coelomic corpuscles.

The different lobes of Nephridium consists of gland cells. It bears intracellular canals or ducts inside. The ducts of different lobes opens into wide central canal. Central canal opens into vesicle after 1 and 1/4 turn.

Process of excretion 0r Physiology of excretion - 

• The ciliated organ is completely separated from the nephridium in adult stage. Ciliated organ has no excretory function in adult but it manufacture coelomic corpuscles of haemocoelomic system.
• The nephridium proper is excretory in function. It serves to eliminate water and nitrogenous wastes from the body. Nitrogenous consists of mainly ammonia. Quantity of urea hence leech is a Ammonotelic.
• Nephridia acts as excretory as well as osmoregulatory organ. Each nephridium is richly supplied with branches of haemocoelomic channels. The gland cells absorb excretory products from the haemocoelomic fluids. Excretory fluid is finally collected in the vesicle of nephridium and lastly expelled out through nephridiopore.
• Nephridia are osmoregulatory in function because maintained water balance in the body. The osmotic pressure of body fluid is always higher than the surrounding water. Hence water enters into the body continuously.
• The Nephridium removes water from the body. Leech has special mechanism in the epidermis, to absorb Na + Cl ions to opens at the loss in metabolism.