'r*:^- 



In sheltered places, such as under fallen logs, slugs (Limax) deposit their glistening eggs. 

 (Massachusetts. Lynwood M. Chace) 



world. Some of these ways of life date back at least 

 six hundred million years. 



The class Pelecypoda is named for those members 

 that can leap or tether themselves or burrow or plow 

 or creep, for these are the ones in which the ventral 

 portion of the body is extensible as a bladelike foot 

 (pelekys. a hatchet, pes, podos, a foot). No pelecy- 

 pod has a head — not even a radula to represent this 

 region of the digestive tract. Essentially everything 

 has been sacrificed to the various ways in which 

 the clams and their kin draw food particles from the 

 water into the mantle cavity, between the two great 

 fleshy mantle lobes that secrete the two-part hinged 

 shell. 



The hinge of the shell introduces a special prob- 

 lem in growth, since nothing must interfere with the 

 ability of the two valves to gape or to close while the 

 animal is exposed to enemies. Consequently the 

 hinge lags behind the growth of the shell, and enlarge- 

 ments are added eccentrically. 



As in all mollusks, the outer surface of the mantle 

 (particularly its edge) secretes the materials and 

 lays down the molecules in precise orientation. This 

 masterful accomplishment produces an outer horny 

 layer (periostracum) protecting the limy shell from 

 external erosion. Under this, calcium carbonate crys- 

 tals lie in tightly packed prisms, forming most of the 

 thickness of the shell. The innermost layer, to which 



The large slug ArioUmax columhianus reaches a length of 8 inches in the humid coastal regions 

 around Paget Sound, Washington. ( Ralph Buchsbauni ) 



