94 PHYLUM MOLLUSCA 



the shell, but when it is walking or feeding it protrudes its head 

 and foot. The visceral mass, containing all its viscera, is always 

 covered by the shell and has thus its exact shape ; that is, it is 

 an elongated cone which has suffered a dextral twisting so as to 

 form a closely coiled spiral. As a matter of fact, however, it is 

 the visceral mass which has been primarily twisted ; the shell is 

 twisted because it covers the visceral mass. If the spiral were to 

 be imagined uncoiled and extending straight up above the foot, 

 the apex would be the uppermost and the foot the lowermost por- 

 tion of the body ; the apex is thus, morphologically, the dorsal 

 and the foot is the ventral aspect of the animal. 



As in the pelecypod, the visceral mass is inclosed in a mantle, 

 which is a fold of the dorsal integument, but unlike the pelecypod 

 it is a single fold and not a double one. This fold falls about and 

 covers the visceral mass on all sides, as does a thimble the fingei 

 it is on, and secretes the shell on its outer surface. The ventral 

 edge of the mantle is provided with muscles, so that it can be 

 protruded beyond the mouth of the shell or retracted within ii 

 This edge is called the collar. Find it in your specimen. On the 

 right side of the animal note the deep notch and the round hole 

 in the collar. This is the respiratory pore, which opens into the 

 respiratory chamber. This chamber is the mantle cavity. Probe 

 it gently and determine its extent. The animal being terrestrial 

 has no gills, but respires by means of a lung, which is a highly 

 vascularized portion of the wall of the mantle cavity. In a live 

 animal note its power to open and close the respiratory opening. 



The foot of the animal forms a broad creeping disk, adapted 

 for locomotion on flat surfaces. Its wavelike undulations may 

 be observed by causing the animal to walk over a glass plate. 

 The head, which is wanting in the pelecypods, forms the anterior 

 end of the animal and bears two pairs of hollow, retractile ten- 

 tacles, the posterior pair carrying each an eye at its extremity. 

 The mouth of the animal is between and a little below the base of 

 the anterior pair of tentacles. Probe it and note the paired, lobed 

 lips. Just beneath the mouth is the broad opening of the pedal 

 slime gland. Probe it and note the extent of the gland. On the 

 right side of the head is a straight groove which extends to a 



