A LAND SNAIL 115 



in very hot (not boiling) water, in order to loosen the shell; 

 twist it then out of the shell ; this must be done very gently, 

 otherwise 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, uncalci- 

 fied layer, which gives the color to the shell. 



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 cavity. Put the blow-pipe through the respiratory pore 

 and blow into the- mantle cavity. It will be seen to extend 

 from the collar to the posterior side of the first whorl. Exam- 

 ine the mantle wall with a hand lens and against the light. 

 The network of blood vessels will be seen, which constitutes 

 the lung. On the hinder border of the mantle cavity note the 

 kidney, an elongated, light-colored, triangular organ ; just in 

 front of it and beneath it, i.e., between it and the mantle cavity, 

 is the heart within the pericardium ; note the two chambers of the 

 heart, the dorsal auricle and the more ventrally placed and 

 larger ventricle. Back of the kidney is the dark-colored liver, 

 which, with the intestine and the light-colored reproductive tract, 

 occupies the remainder of the coils of the spiral. Note the 

 rectum, a broad tube on the inner (right) border of the mantle 

 cavity going to the anus. Cut a small hole in it, and through 

 this pass a probe to the anus. 



The mantle cavity. Lay this open in the following way : with 

 fine scissors cut through the collar at the respiratory pore ; 



