62 Invertebrate Zoology. 



seen. It is a large crescentic area, often slightly depressed. 

 On the inside of the visceral mass, and extending somewhat 

 parallel to the nidamental gland, is the large columellar mus- 

 cle, by means of which the animal retains its hold upon the 

 columella of the shell. 



Make drawings of the animal as seen from in front and 

 as seen from the outer and inner sides. 



The Anatomy of the Mantle Cavity. With a pair of 

 scissors slit open the roof of the mantle chamber along a 

 median line extending parallel to the upper edge of the 

 gill. Reflect the flaps and observe, on the inner side of 

 the left, the extent of the single large lamellate gill. What 

 is the arrangement of the branchial lamella ? Immediately 

 below the gill is a smaller organ, somewhat similar in out- 

 line and structure. It is the osphradium or olfactory organ. 

 On the inner surface of the right flap the external opening 

 of the rectum will be noted. It is at the apex of a short 

 anteriorly directed tube. 



If the specimen is a female, the external opening of the 

 oviduct will be found at the apex of an elevation near, though 

 a little lower than, the anal papilla. Pass a flexible probe 

 (a guarded bristle) into the opening. 



Males are somewhat smaller than the females, and may 

 be readily distinguished by the intromittent organ. Lead- 

 ing out from the base of the penis, and inclining towards 

 the right, until it finally reaches the groove made by the 

 union of the roof of the mantle chamber to the body, is a 

 clearly defined ridge which marks the course taken by the 

 vas deferens. Trace the vas deferens to the upper portion 

 of the mantle chamber. 



In both sexes a large gland, the hypobranchial or adrectal 



