196 SYNOPTIC COLLECTION. 



MOLLUSCA. 



Section 7. PELECYPODA. 



The arrangement of the molluscs in the Synoptic Col- 

 lection is governed by the same principle that controls the 

 classification of the preceding subkingdoms. We con- 

 sider first, primitive ancestral Pelecypods and the early 

 stages of living forms ; secondly, the specialization of 

 adults. Since a fleshy animal antedates, as a rule, a skel- 

 eton-bearing animal, as we have seen in the Protozoa, 

 Porifera, and Coelentera, and since, also, fleshy parts are 

 not usually preserved in the geologic formations, we turn 

 to the embryonic and larval stages of existing species to 

 determine so far as possible the characters of the ances- 

 tral fleshy forms. On the other hand, since the skeleton 

 or shell when made, is a comparatively sure guide to the 

 structure of the soft parts, we regard with special interest 

 the remains of the primitive Mollusca in the Protozoic 

 and the Palaeozoic strata. An early stage of the pres- 

 ent Pelecypod larva is the trochophore (PI. 368). It is 

 a little fleshy creature whose distinctive features are a cil- 

 iated locomotive ring (the velum) in front of the mouth, 

 and a tiny sac or shell gland on the dorsal surface. This 

 sac is simply a portion of the outer wall turned inward, 

 but it plays an important part since it is soon everted 

 and at once the shell begins to form on its surface. The 

 latter is secreted extremely early in the life of the embryo, 

 therefore it is inherited, and of value phylogenetically. 

 At first it is shaped like a tiny plate or cap. and therefore 

 the ancestor from which the Mollusca descended prob- 

 ably possessed such a shaped shell. In the case of the 

 developing Pelecypod the tiny plate forms into two parts 

 or valves, while in the Gastropod the cap becomes a cone 

 and later a new shell is formed which in most cases 

 becomes a spiral. 



