256 '■"^ ANIMAL KINGDOM 



posterior adductor muscle, below the intestine. An additional pair of 

 pedal ganglia (which in the gastropods have moved forward to join 

 the brain) are deeply embedded in the loot of pelecypods. Nerve cords 

 connect these various components. Sense organs are limited to scattered 

 chemoreceptors on the palps and siphons. Touch and temperature 

 sense endings are probably present along the mantle edges. A tew pele- 

 cypods have eyes but they are at the mantle edge, never on the head. 



111. Other Pelecypoda 



Many clams, including the steaming clam, Mya arenaria (Fig. 14.11), 

 live buried in the sand and mud like Venus. Others, such as the cockle, 

 Cardium edule (Fig. 14.2), jump over the bottom with quick movements 

 of the foot. Mussels and oysters are attached to rocks and pilings. The 

 common mussel, Mytilus eduUs, attaches by a cluster of strong threads 

 secreted by a gland at the base of the foot. Oysters cement one valve to 

 the bottom. 



The edible oyster (several species of the genus Ostrea) is harvested 

 by the most intensive and thoroughly regulated fisheries in the world. 

 Along the eastern seaboard of the United States, for example, wherever 

 the bottom is especially suitable for oysters it has been surveyed and 

 rented to various fishermen by the states. Once a fisherman rents a given 

 area, he is entitled to rent it for the rest of his life, and to pass on the 

 privilege to his heirs. Each oysterman manages his own "land" to pro- 

 duce as many oysters as possible. Every year boatloads of old shells are 

 scattered about to serve as possible sites for the attachment of larval 

 oysters. Once larvae are attached to these loose shells, they may be 

 moved about several times, inshore each summer for maximum growth, 

 offshore in winter for protection, and finally to premium spots where 

 they develop the best flavor for marketing. Oysters mature in three to 

 five years. Curiously, most of the young are not produced by the older 

 oysters of the fishery, but come from scattered populations along the 

 rocky shores and especially in the mouths of rivers where the water is a 

 little less salty. These "wild" oysters produce enormous numbers of 

 young that drift offshore and eventually settle to the bottom. 



Oysters are hermaphroditic; an individual may be a male for a few 

 years, and then become a female, but it is never both sexes at once. 

 The American oysters shed both eggs and sperm into the water where 

 fertilization is left to chance. The gametes shed by one individual enter 

 the siphons of other oysters, causing them to release their gametes also, 

 and soon the entire bed has been triggered. 



The pearl oysters (species of the genus Meleagrina) are found in 

 warm seas, especially around Japan (Fig. 14.11). Theoretically any 

 pelecypod can produce pearls, and many species such as the common 

 mussels and oysters often do, but only the pearl oysters produce pearls 

 of consistent high quality. The formation of a pearl is a reaction of 

 self-defense. If a small foreign body should become lodged between the 

 mantle and the shell, a layer of shell is secreted around it to seal it off. 

 If the foreign body should be buried in the flesh of the mantle, shell is 



