246 ZOOLOGY 



in two sub-equal valves, right and left, or may be in one piece, 

 in which case it is usually coiled or spiral (Fig. no). The 

 bivalved types are able to open and close the shell, and the soft 

 parts are capable of protrusion from the partly opened shell. 

 This latter power is much more pronounced in the univalved 

 types (e.g., snail). The fundamental bilateral symmetry is 

 obscured in the more sluggish forms, but is very decided in such 

 active animals as the squid and some of the bivalves. 



e,eL.cu.— 



Fig. 109. Shell of a Bivalve MoUusk, inner surface. ad.a., depression showing the attach- 

 ment of the anterior adductor muscle; ad. p., posterior adductor muscle; A, hinge with teeth; It 

 attachments of the ligaments; p, pallial line, marking the attachment of the mantle muscles; s, 

 the pallial sinus, marking the attachment of the retractor muscles of the siphon; u, umbo or beak. . 



Questions on the figure. — Which is the dorsal aild which the ventral portion 

 of the shell? Is this the right or left valve? What is the effect of the contraction 

 of the adductor muscles? What is the value of the teeth on the hinge? To what 

 point in the shell of the snail does the umbo correspond? 



One of the most interesting points of differe9ice among the 

 members of the group is the degree of development of the 

 "head." In the bivalves (lameliibranchs) there is a very 

 slight cephalization, or collection of special organs about the 

 anterior end. For this reason they are often called Acephals. 

 In the gasteropods (snails, etc.) and cephalopods (squid), on 

 the other hand, the head is well developed both as to special 

 mouth parts and nervous organs. 



