Phylum Mollusca and Minor Eucoelonwte Phyla 477 



grain and thus form a pearl. This may occur in fresh-water clams, 

 but is much commoner among oysters. In Japan, pearls are cultivated 

 by the careful insertion of materials; after a period of time, the pearls 

 are harvested. 



At one time, the clams of the Mississippi drainage were the richest 

 and most varied in number of any place in this hemisphere. The alka- 

 line waters probably account for this variety of forms. Due to over- 

 fishing, the clams were at one time in danger of depletion, but careful 

 study and control have done much to bring back the supply. They are 

 important as a source of pearl buttons. 



A less desirable member of this class is the Teredo or shipworm. 

 This tiny bivalve burrows into wooden structures and causes great 

 damage. In coastal areas, these shipworms cause great property loss 

 to pilings and wharves. 



THE CLASS CEPHALOPODA 



There is a tendency among the molluscan groups toward a reduc- 

 tion of the amount of the protective shell. This reduction is carried to 

 an extreme in this class where the squids and octopuses have merely 

 a supporting internal structure. Instead of relying upon the sure pro- 

 tection of the shell, these cephalopods rely upon speed, a well-developed 

 nervous system, good sense organs, and general alertness for their sur- 

 vival. 



All of the cephalopods are strictly marine forms and are of several 

 diverse types: the squids, the octopuses, the chambered nautilus, and 

 the argonaut. The name cephalopoda or head-footed refers to the fact 

 that the head and foot are closely associated, with the foot surrounding 

 the well-developed head and then being divided into arms. The arms 

 are united by a web at their bases and in most forms bear suckers. 

 The main portion of the body may be bulbous as in the octopus, or 

 elongated as in the squid, and is surrounded by the mantle. Except 

 for the chambered nautilus they all have ink glands which produce a 

 dark fluid. Only the nautilus has a shell into which the body may be 

 withdrawn ; the female argonaut has an external shell but it is secreted 

 by the arms rather than by the mantle. 



Loligo, a Squid. — The members of the genus Loligo are small 

 squids about 10 inches long which are found along both of our coasts, 

 often in very large numbers. They are eaten by fish, used for bait by 

 fishermen, and often are popular with sea food advocates. 



