CLASS PELECYPODA 
HIS is a large and important class of the Mollusca, 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 would 
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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