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| Excretory systems in crustacea |
By:
Snigdha |
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The renal organs of Crustacea are two pairs of glands, the aniennary and the maxil!ary glands. Both pairs of glands rarely function together, but one succeeds the other. Generally the antennary glands function as kidneys in the larvae while the maxillary glands in the adult. But, the reverse is true in the orders Decapoda (including prawns), Amphipoda, Schizopoda and Dichelopoda.
The excretory system of the adult Palaemon consists of (I) a pair of antennary or green glands, (2) a pair of lateral ducts, (3) an unpaired renal-sac, and (4) the integument.
Antennary or green glands
One antennary gland is enclosed within the coxal segment of each antenna. It is an opaque white gland as big as a pea-seed or gram-seed, and comprises three parts—(i) the end sac, (ii) the labyrinth or glandular plexus, and (iii) the bladder. The bean shaped end-sac is the smallest part lying in between the bladder and the labyrinth. Internally, its cavity contains a large blood-lacuna. Its wall is made of two layers and projects in the form of radial septa into the central —cavity. The outer thick layer of the wall consists of connective tissue containing numerous small blood-lacunae, while the inner thin layer consists of large excretory epithelial cells. The labyrinth or glandular plexus is relatively larger than the end-sac and lies on its outer side. It consists of numerous narrow, branching and greatly coiled excretory tubules embed4ed in a mass ofconnective tissue containing blood-lacunac. The tabules are lined by a single layer of large excretory epithelial cells, and they open by a single aperture into the end-sac, but by many apertures into the bladder.The bladder is the largest of all, lying on the inner side of the end-sac. It is a thin-walled sac made of a single layer of excretory epithelial cells. Its inner wall is prolonged as a short excretory duct or ureter, which opens to the outside through small rounded renal aperture, situated on a papilla on the inner surface of the coxa of the antenna.
Lateral ducts
A narrow lateral ducts runs posteriorly from the bladdcr of each antennary gland. The lateral ducts of both the sides are connected by a transverse Fig. 2-31 Excretory organs of connective, in front of the brain. The two ducts run backwards along the oesophagus to open into the renal-sac.
Unpaired renal-sac
It is a large, thin-walled sac, lying above the cardiac stomach just beneath the carapace and extending posteriorly up to the gonads. Its wall is’ made. of a single layer of flattened excretory epithelial cells.The complex, nephridia-like anten nary glands extract nitrogenous waste products and excess water from the blood in much the same manner as the vertebrate kidneys do. The end-sac excretes compounds of ammonia, but uric acid and other nitrogenous compounds are excreted by other parts. The excretory fluid is collected first into the bladders and then expelled out through the renal apertures.
Integument.
The non-living chitinous covering or integument is cast off at each moult, when the nitrogenous products secreted by the body and deposited on the integument are also expelled. Thus integument is believed to be an important excretory organ.
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