CLASS PELECYPODA 



THIS is a large and important class of the MoUusca, compris- 

 ing an extensive group of animals, which vary widely in 

 structure. Next to the Gasteropoda, it is the largest class of the 

 phylum in number of genera and species. The name indicates a 

 "hatchet-shaped" foot, but in many cases it is a misnomer. 

 Acephala, meaning " without a head/' is also employed for this 

 class, and is, perhaps, a better term. Lamellibranchiata, referring 

 to the type of gills prevalent throughout the division, is another 

 name often used. The popular name " bivalves " is an altogether 

 correct one, for all the animals of this class have two shells. To 

 the Pelecypoda belong the oysters, clams, scallops, mussels, and, 

 in short, all the bivalve mollusks (the word " valve " meaning 

 " shell "). The organization of a pelecypod is entirely similar to 

 that of a gasteropod or a chiton in its fundamental or essential 

 plan, but it differs widely from both in matters of detail. If the 

 student will keep in mind the general principles of gasteropod or 

 the amphineuran structure when he dissects his first pelecypod, he 

 will quickly see that the latter is about the same as a chiton woul'd 

 be were it folded over from a longitudinal median line as axis 

 along its back, so that the opposite mantle-edges would meet, and 

 its various shell-plates would unite upon either side into a single 

 valve ; or it is very like a gasteropod made symmetrical, and cov- 

 ered upon its two sides by separate shells instead of covered over 

 its top by one shell. The loss of head, eyes, and tentacles, the 

 substitution of labial palps, the extension and greater develop- 

 ment of the gills, the modification of the foot and mantle, and the 

 presence of certain special glands in the Pelecypoda become mere 

 matters of anatomical detail. 



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