104 INVERTEBRATE ZOOLOGY 



The foot of the anirnal forms a broad creeping disc, adapted 

 for locomotion on flat surfaces. Its wave-like undulations may 

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

 The head, which is wanting in the lamellibranchs, forms the ante- 

 rior end of the animal and bears two pairs of hollow, retractile 

 tentacles, 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 depression just behind the base of the anterior 

 tentacle. This depression is the common genital pore, the animal 

 being hermaphroditic. The anus is a small opening just beneath 

 the respiratory pore at the end of a deep groove. It is not 

 easily observed from the outside. 



Note the asymmetry of the animal. Its spiral twist has been 

 the cause of the loss of the primitive bilateral symmetry of the 

 visceral mass and shell. They are not borne squarely above 

 the foot, but obliquely and to the left. The respiratory pore 

 (i.e., the opening of the mantle cavity) and the anus have not 

 a median posterior position, as must have been the case in the 

 primitive ancestor of the animal, but have suffered displace- 

 ment to the right side. Other instances of asymmetry will be 

 noticed as the dissection proceeds. 



Exercise 1. Draw a side view of the animal seen from the right 

 side as it appears when it is moving and when the head 

 and foot are out of the shell, and label the parts above 

 mentioned. 



Exercise 2. Draw a similar sketch of a front view of the animal. 



Remove the dead animal from its shell in the following way : 

 place it for five minutes in strong alcohol, or for half a minute 



