114 INVERTEBRATE ZOOLOGY 
The foot of the animal 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 pelecypods, 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 
