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. 



^ ifll of a Bivalve Mollusk. inner surface, ad. a., depression showing the 



e anterior adductor muscle; ad. p., posterior adductor muscle; h, hinge 



Attachments of the ligaments; p, pallial line, marking the attachment of 



- !es; s, . the pallial sinus, marking the attachment of the retractor 



of the siphon; u, umbo or beak. 



Questions on the figure. Which is the dorsal and which the ventral 

 -hell? Is 'this the right or left valve 3 What is the effect 

 ictioj;,,. f h9 - : ; muscles? What is the value of the 



""ill of the snail does the 



