264 COLLEGE ZOOLOGY 



(p. 251). Pearl- fishing should also be noted. Pearls are pro- 

 duced by secretions of the mantle around a foreign substance, 

 such as a grain of sand or a parasitic worm. The Pelecypoda 

 of the Persian Gulf yield the finest pearls. 



One bivalve, the ship worm, Teredo navalis (Fig. 190), is in- 

 jurious to ships and piles. It burrows into the wood with its 

 shell, sometimes to a depth of two feet. 



6. Class V. Cephalopoda 



The Cephalopoda are the squids, octopods, and nautili. They 

 are constructed on the same fundamental plan as other mollusks 

 (Fig. 172, III), but are very different m form and habits. 



a. The Common Squid — Loligo 



Loligo pealii (Fig. 191) is one of the common squids found 

 along the eastern coast of North America from Maine to South 

 Carolina. It probably lives in deep water during the winter, 

 but about May 1 it enters shallow water in large schools to lay 

 its eggs. Squids are of some economic importance, since they 

 are used as food by Chinese and Italians, and as bait for line and 

 trawl fishing. They feed on small fish, Crustacea, and other 

 squids, and in turn furnish food for cod and other large fish. 



Anatomy and Physiology. — The body of Loligo is spindle- 

 shaped. When swimming through the water the morphological 

 ventral surface is usually anterior (Fig. 191, V); the dorsal sur- 

 face is posterior (D); the anterior surface is dorsal (^4); and 

 the posterior surface is ventral (P). The skin may change color 

 rapidly; sometimes it is bluish white, at others, mottled red or 

 brown. 



The foot consists of ten lobes (Fig. 191, 5, 6, 7) and a, funnel (3). 

 Eight of the lobes are arms (5, 7) and two are long tentacles (6). 

 The inner surfaces of both arms and tentacles are provided with 

 suckers. The arms are pressed together and used for steering 

 when the squid swims, but when capturing prey the tentacles are 



