Figure 2: Cleavage in Mactra chinensis (from Medvedeva and Malakhov, 1983). 



A — stage with 2 blastomeres, lateral view; B — stage with 8 blastomeres, view from 



animal pole; C — stage with 8 blastomeres, view from animal pole; D — stage with 16 



blastomeres, view from vegetal pole. Legend given in text. 



bivalve D. polymorpha, only trochophore larvae are free living (Meisenheimer, 

 1901; see Figure 5). 



The prototroch — a broad ciliary circlet — is the main organ of locomotion 

 of the trochophore larva. On the animal pole of the trochophore an apical tuft 

 of cilia appears, under which lies a thickening. of the ectoderm (aboral organ). 

 The mouth is situated under the prototroch and leads to blind gut. The terminal 

 part of the gut adjoins the ventral wall of the trochophore where, later, the hind 

 gut and anus form. 



Veliger 



The trochophore transforms into a veliger, which has a more complex 

 structure (Figures 7 and 8). The animal pole of the trochophore develops into 



