220 ELEMENTARY ZOOLOGY. 



242. The Nervous System. There are some features of the nervous 

 system that are fairly constant for the whole branch. There are 

 three pairs of principal ganglia or nerve centers (Fig. 70) : One pair 

 in the region of the mouth above the gullet (cerebral) ; one in the 

 foot (pedal) ; and one, posterior to either of these, controlling the 

 internal organs and functions (pleura-visceral). There may be 

 other ganglia, but these are the foundations of the system. They 

 are connected by means of loops of nerve fibres (see Fig. 70). In the 

 cephalopods these loops are very short and the ganglia are brought 

 together into a well-organized brain in the head. This massing of 

 the ganglia in one place is associated with the good development of 

 the head-organs in the squid, and is a measure of its high place 

 among the invertebrates. 



FIG. 69. Shell of a bivalve mollusk, inner surface, ad. a., depression showing 

 point of attachment of the anterior adductor muscle; ad.p., posterior adductor muscle; 

 h, hinge, with teeth; /, attachments of the ligaments; p, pallial line, marking the 

 attachment of the muscles of the mantle; s, pallial sinus, marking the attachment of 

 the retractor muscles of the siphon ; u, umbo or beak. 



Questions on the Figure. Which is the dorsal and which the 

 ventral portion of the shell? Is this the right or left valve? Is 

 there any other fact that you will have to determine before you 

 can answer? What are the adductor muscles? What is their 

 function? Of what use are the teeth on the hinge? What is the 

 siphon? To what point in the shell of the snail does the umbo 

 correspond? Evidences? 



