A LAND SNAIL 95 



depression just behind the base of the anterior tentacle. This de- 

 pression is the common genital pore, the animal being hermaph- 

 roditic. 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 vis- 

 ceral mass and shell. They are not borne squarely above the foot, 

 but obliquely and to the left. The respiratory pore (that is, 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 displacement 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 in 

 very hot (not boiHng) water, in order to loosen the shell ; then twist 

 it out of the shell ; this must be done very gently, or the animal 

 will be torn. 



Exercise 3. Draw the shell showing its opening on the right. 



Break off a portion of the edge of the shell and examine the 

 broken edge with the aid of a hand lens. Note the three layers 

 which compose the shell : the inner pearly layer, which has been 

 secreted by the entire surface of the mantle ; the thick middle 

 layer and the thin outer layer, or periostracum, which have been 

 secreted by the collar. The periostracum is a horny, uncalcified 

 layer, which is usually more or less worn off. 



The Internal Organs. Take the snail, deprived of its shell, in 

 the hand, and remembering that the outer side of each whorl of 

 the spiral is on the left side of the animal and that the inner side 

 of the whorl is on the right, observe the extent of the mantle 



