MOLLUSCA 



73 



where the Unionidae are very common, all classes of fresh- water 

 fish are used by the members of the family, including the 

 ganoids. 



In the sea the Pelecypoda are spread as larvae, which 

 are subject to drift in tidal and ocean currents. The larva is 

 known as a trochophore, and its development and structure 

 are interesting and important. During segmentation changes 



Stomodeum 



Apical 

 ganglion 



Proctodeum 



Mouth 



Stomach and 

 dig. gld. 



Pericard. 

 cavity 



Anus 



Otocyst Protoneph. 



FIG. 36. Development of trochophore larva of a bivalve mollusc. The 

 endodermal area of the alimentary canal is distinguished from the 

 ectodermal. 



occur which are very similar to those of the egg of Annelida. 

 A blastula is converted into a gastrula, and the blastopore of 

 the latter is either converted into the mouth or marks the 

 place where the mouth appears. It is carried inwards by a 

 stomodeal imagination. The foot is produced immediately 

 behind the mouth, thus from the region of the posterior (dorsal) 

 lip of the blastopore. The anus is developed later by a back- 

 ward growth of the enteron, meeting and fusing with the 

 ectoderm behind the foot. A small ectodermal invagination 

 is formed to meet the enteron, and this is termed the procto- 

 deum. The alimentary canal thus consists of an anterior 



