Physiology of Finfish and Shellfish

2.3.8. Mollusca

The anatomical form of the renal gland varies from one class of mollusks to another, but a common plan is clearly evident. The renal gland is a relatively wide tube opening from a sac (the pericardium) surrounding the heart , at one end, and to the mantle cavity (effectively to the exterior) at the other. There is a single pair of renal glands; in some forms one member of the pair may be reduced or absent. Clams have the simplest arrangement.

Like most other molluscs, the excretory organs of bivalves are nephridia . There are two nephridia, each consisting of a long, glandular tube, which opens into the body cavity just beneath the heart, and a bladder . Waste is voided from the bladders through a pair of openings near the front of the upper part mantle cavity, where it can easily be washed away in the stream of exhalant water.

Blood is forced through the walls of the heart into the pericardium. From there, it passes into the kidneys where wastes are removed, producing urine. The paired kidneys (nephridia) are looped with an opening into the pericardium and another into the suprabranchial chamber. The kidneys may be united. Bivalves also possess pericardial glands lining either the auricles of the heart or the pericardium; they serve as an additional ultrafiltration device.

Paired excretory organs lie on either side of the heart and a "kidney" in each extracts waste products and discharges them, via a bladder, into the gill passages and hence to the exterior. collect fluids from the coelom and excha The vast majority of the excreted ammonia originates from the catabolism of proteins and amino acids.