13 



The gut wall lacks muscles and the movement of food in the gut is 

 accomplished by cilia. In the larvae of some species, behind the anal opening, 

 which is surrounded by short cilia, there is a postanal tuft of dense 1 5 |4.m long 

 cilia (Figure 10). The postanal tuft facilitates removal of fecal matter from the 

 mantle cavity. 



Figure 10: Veliger of Ostrea edulis L. (from Waller, 1981). 



Posterior view: b — border between prodissoconch I and II; pat — postanal tuft of cilia; 



rf — rudiment of foot; v — velum. 



Respiration : A specialized respiratory apparatus is absent in the larvae of 

 bivalves. Oxygen is supplied and carbon dioxide eliminated through difftision. 

 Since difftision is facilitated by a flow of water, the various locomotory hy- 

 drokinetic structures of the larvae, in addition to other ftinctions, also accom- 

 plish respiration. These are, the pre- and postoral ciliary tufts, and also the 

 ciliary tracts of the esophagus and gut. Respiration is further assisted by cilia 

 situated along the margin of the mantle cavity, including the region of ftiture 

 gill formation. 



Transport of substances : The larvae of bivalves lack a circulatory system. 

 Nor do they possess a branched gastrovascular system, which could replace a 

 circulatory system. The transport of substances from one organ to the other and 



