240 



ZOOLOGY. 



2. The organ of locomotion is a muscular thickening of the 

 body, called the foot, which is variously modified. 



3. A thickened dorsal fold of the body wall, called the 

 mantle, is usually present. This encloses a space, external to 

 the body, known as the respiratory chamber. 



4. The mantle secretes in many cases a calcareous shell, -at 

 first single and symmetrical, but usually becoming either spiral 

 or separated into a right and left valve. 



5. The central nervous system usually consists of three sets 

 of ganglia: (i) the cerebral or " brain," above the mouth, (2) 

 the pedal, in the foot and connected with the cerebral by 

 nerves, and (3) the visceral, also connected with the brain 

 by a pair of nerves (Fig. 36). 



6. Except in the headless forms (Acalephs} a tooth-bear- 

 ing ribbon, the odontophore, is found in the mouth. 



-atf.jb. 



FIG. 107. Shell of a Bivalve Mollusk, inner surface, ad. a., depression showing the 

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

 muscles of the siphon; , 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? 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? 



