Coelomoducts of Molluscs

In Molluscs, like in Crustacea nephridia are absent. But certain of the larval pulmonates do possess protonephridia suggesting that they have been secondarily lost in molluscs. The excretory system is much more or less similar in all the groups of molluscs. In molluscs it is assumed that the two coelomic cavities meet dorsally to enclose the heart and their walls proliferate into germ cells. Their cavities through further differentiation give rise to gonad anteriorly, pericardial canal centrally and gonoduct posteriorly. The final segment, in addition had an excretory function. In Aplacophora the coelomoducts of adults contain a pair of tubular structures, leading from the coelomic cavity to the outside and primitively constitute the genital ducts. But in other molluscs modifications have arisen in the subsequent lines:

 i) There is a separation of genital and excretory organs.

ii) There is the development of fixed degree of asymmetry.