40 STUDIES OF ANIMAL TYPES 



until each one stands by itself as a minute dot. At every 

 successive contraction and expansion of the pigment cells, 

 in a living specimen, blushes of different hues may be 

 observed passing over the body. 



Note that the head seems to sit down in a collar of thick 

 skin, or flesh. This is the anterior end of the thick, fleshy 

 mantle that covers all of the body except the head. Is the 

 head grown to the mantle? How does the mantle com- 

 pare with the mantle of the clam ? Beneath the head, note 

 the siphon. Under ordinary conditions, the water, for re- 

 spiratory purposes, passes in and out through the aperture 

 between the mantle and the neck. But when the animal 

 desires to swim the mantle contracts and closes up the 

 opening about the neck, and the water is forced out through 

 the siphon. 



Make a written comparison of the clam and squid. 



XIII. THE HABITS AND LIFE HISTORY OF A POND 



SNAIL 



Materials. Specimens of Physa (alive) . 



Directions. (The two common pond snails, Limnea and Physa, 

 are abundant in many ponds. They may be dipped up and kept in 

 jars of water almost any length of time, if some pond scum and 

 lettuce leaves are added for them to eat. The following outline 

 applies especially to the smaller one, Physa.) 



A. STUDY OF A LIVING SNAIL. Note the color of the 

 shell. What is its shape? Is it a flat or an ascending 

 spiral? Does it coil to the right or left? Note that it 

 consists of one piece or valve. Compare it with the clam. 

 Watch some of the individuals climb up the sides of the 

 jar. They progress swiftly, with a smooth, gliding move- 



