MOLLUSCA 257 



and mussels that are used for food. Less frequently snails and 

 squid are eaten. 



Pearl buttons, knife handles, and other objects are made 

 from the "mother-of-pearl" shells of mollusks. Pearls, one of 

 the most prized ornaments, are produced by a number of species, 

 notably fresh-water clams and the marine pearl oyster. 



Squid produce cuttle bone and sepia, and are used for codfish 

 bait. 



The oyster industry and the clam-shell industry have become 

 so extensive that they have ceased to be merely "fisheries." 

 Promiscuous fishing, if profitable, always results in threatening 

 the whole industry. The next step is to limit fishing or devise 

 means of encouraging the growth of the organisms, or both. 

 This has been done for the oyster and is beginning for the clam. 

 The steps involved are to favor the young by artificial means, to 

 give them suitable places to settle, to furnish them with food, 

 and to protect them from enemies and unfavorable environment. 

 This is a form of "farming," rather than fishing. Coupled with 

 legal limitations upon fishing such measures make for steadiness 

 of supply, and should be encouraged in every way possible. 



302. Classification. The following are the principal classes: 

 Class I. Pelecypoda (hatchet-footed) or Lamellibranchiata; (Mussels, Oysters, 

 etc.). Lamellibranchs are mollusks 'in which the fundamental bilateral sym- 



FIG. 117. 



FIG. 117. Ensis amcricanus, the razor clam. From Verrill, after Gould. 



Questions on the figure. Where is the hinge, the umbo, etc. ? Trace the lines 

 of growth and compare with other figures of bivalves. 



metry is shown in the right and left valves of the shell secreted by a bilobed 

 mantle, and in some of the internal organs. There may be one or two adductor 

 muscles. The head is undeveloped. The ventral body region is differentiated 

 into a muscular foot, shaped like a plow-share. The gills are in sheets (see 

 291) usually two on either side, and are suspended in the mantle cavity. Paired 

 labial palps occur about the otherwise unspecialized mouth. The three pairs of 

 ganglia the cerebro-pleural, the pedal, and the visceral, are usually well sepa- 

 17 



