Chap. 31 MOLLUSKS — specialists in security 633 



except as they cUng to boats, driftwood, and floating plants, to the bodies of 

 fishes, seals, and whales; and on land to the feet of birds. The striking excep- 

 tions are the free-swimming squids that range the seas. Sense organs are not 

 highly developed in mollusks, the tactile sense, and the eyes of land snails, 

 scallops, squids and octopuses excepted. Great aggregations of marine snails 

 and mussels are common. On land, slugs and snails congregate in moist places 

 and about decaying tissues, but there is no such variety of responses and social 

 relationships as in arthropods. The periwinkles and blue black mussels that 

 cling to rocks between the tide lines are expressive of the monotony of relative 

 safety and endurance that accompanies their survival. Security is expensive. 



Oysters feed upon microorganisms from the muck and water of the bottom 

 and in turn are consumed by starfishes, oyster borers and mankind. In open 

 sea, enormous numbers of squids follow and feed up on schools of herring and 

 other fishes. Toothed whales attack the giant squids. Part of a giant squid's 

 arm, eighteen feet long, was once taken from a whale's stomach. 



Mussels, clams, scallops, oysters, and various kinds of snails including 

 abalones, are all used for human food. In North America, the "American 

 oyster," Crassostrea virginica, that is cultured along the eastern coast, brings an 

 annual income of millions of dollars. The native oyster (Ostrea km da) of the 

 Pacific coast is commercially less important. It is small and when shucked 

 there may be 1600 to 2000 in a packed gallon. In late years, Japanese and 

 eastern American oysters have been introduced on the Pacific coast and are 

 thriving especially in the northwest. Scallops are harvested on both coasts but 

 to no such extent as the common oyster. Abalone steaks familiar in California 

 markets, though little known outside the state, are slices of the muscular foot 

 of this large marine snail whose iridescent shell figures in many collections. 

 Formerly great numbers of pearl buttons were cut from the shells of large 

 mussels of the Ohio-Mississippi River system. That industry has almost dis- 

 appeared since synthetic substances have captured the market. 



The Classes. The five classes of mollusks have Greek names, all but one 

 referring to the shape or location of the foot (Fig. 31.3). These names are: 

 Amphineura meaning double nerve — the chitons; Scaphopoda meaning plow 

 foot — the tooth shells; Pelecypoda meaning hatchet foot — the clams, mussels, 

 and oysters; Gastropoda meaning stomach foot — the snails, conchs, slugs; 

 Cephalopoda meaning head foot — squids, nautiluses, and octopuses. 



Class Amphineura — Chitons 



Chitons are widely distributed mollusks of the many seashores. Their eight 

 overlapping shells are flexibly attached to one another and when a chiton is 

 not clinging to rock it usually rolls up in a ball like an armadillo (Fig. 31.4). 

 There are fossil chitons at least 400 million years old. These also have the 

 typical eight shells, a sign that chitons have survived long and changed little. 



