186 INVERTEBRATE ZOOLOGY 



of producing a definite shell. The foot is lacking and in its 

 place there occurs a longitudinal, ventral, ciliated groove. The 

 evidence seems to indicate that this is a degenerate rather than a 

 primitive type of amphineuran. There are only a few genera of 

 which Chaetoderma, Neomenia, and Dondersia are characteristic. 



Class Acephala (Pelecypoda) 



As is indicated by the name, the members of this class have 

 no specialized head. Many writers on the Mollusca use the name 

 Pelecypoda for the members of this class. This alternative 

 name refers to the hatchet-shaped foot. The Acephala have a 

 shell composed of two lateral valves. Typically the hinge 

 of these valves is dorsal, though its position is modified in many 

 instances. On the valves are usually found conspicuous con- 

 centric lines, known as the lines of growth. The umbo, a slight 

 prominence on the dorsal surface around which the lines of 

 growth are distributed, is the oldest part of the shell. An 

 elastic hinge ligament tends to hold the margins opposite the 

 hinge gaping open. The valves of the shell are closed only by the 

 contraction of adductor muscles which extend from one valve to 

 the other through the body. Immediately within the protective 

 shell lie the right and left lobes ( the mantle, secretions from 

 which form the shell. 



While the entire animal is usually capable of withdrawing 

 completely into the shell, the development of the siphon in some 

 of the burrowing clams is so great that the siphons cannot be 

 pulled into the shell. In the peculiar wood-boring shipworm 

 (Teredo) the shells are diminutive and are modified for boring 

 wood, while protection of the body which is ordinarily afforded 

 by the shell is given by the wooden tunnel within which the 

 worm-shaped animal dwells permanently. Submerged timbers 

 and wooden ships are frequently completely destroyed by the 

 tunnels of shipworms. Another marine group of peculiar habits 

 is that which comprises the rock borers. In these (pholads), the 

 front edge of the shell is used as a rasp to bore into solid rocks. 



The members of this class contain many forms of direct 

 economic importance. The oysters, scallops, and clams are 

 used as food. Shells of the fresh-water mussels and some marine 

 forms are used in the manufacture of buttons. The members 

 of this class play important roles in the food chains of organisms 

 of both fresh water and seas. 



