28 Marine Shells of the Western Coast of Florida 



beach of Sanibel Island, Florida, wind and tide pile up great banks 

 of shells which consist preponderantly of Noetia ponderosa, and 

 from time to time incredible numbers of young Chione cancellata 

 are washed ashore. Less frequently living Cyrtnpleura costata are 

 stranded on the beach by hundreds when some unusual tidal cur- 

 rent has undermined a colony of them. 



Pelecypoda are without distinct heads, and the organs of special 

 sense are not highly developed. The mantle conforms to the shape 

 of the shell. It covers the viscera and contains between its two lobes 

 the gills and mouth parts; its posterior edges are modified to form 

 simple or tubular openings — the siphons — through which currents 

 of water are received into and expelled from the animal's body. Those 

 genera which burrow most deeply have the longest siphons, but in 

 almost every instance the siphons may be completely withdrawn 

 into the shell."' Most pelecypods possess a flexible muscular foot 

 well adapted for digging and limited locomotion; in many genera 

 there is a special gland in the foot whose viscous, adhesive secretion 

 hardens on contact with water to form a byssus. The byssal threads 

 issue from between the valves of the shell and become attached 

 by their distal ends to some solid support. The byssus usually con- 

 sists of fine, silkenlike threads (Atrina and Mytilus), in some species 

 of the genus Area it resembles thin kidskin, or it may become thick 

 and solidified in other species of the genus. In many of the pelecy- 

 pods, a byssus is present in juvenile but not in the adult stage, and 

 in certain genera a byssus appears during the late embryonic develop- 

 ment. 



The food supply of pelecypods is the microscopic plant and ani- 

 mal life of the seas. Currents of water pass through the siphons 

 into the mantle cavity to be depleted of their content of nutriment 

 and oxygen; the waste water, bearing the product of excretion, is ex- 

 pelled in the same manner. 



Most pelecypods are unisexual, some are true hermaphrodites, 



^ Small pinfish in an aquarium have been seen by Perry to nip off the 

 siphons of Tagclus, Tcllina, and Donax as they were projected above the sand 

 Imttom into the water. At the first touch the siphons would be withdrawn, when 

 they reappeared the fish would attack again and eventually the exhausted mol- 

 lusks were exposed and completely devoured. 



