12 LESSON II. 



Child. Their skin expands at the sides of the 

 body, and adheres to the earth, and then they 

 draw themselves on. 



Teacher. This fleshy expansion under their 

 body is full of muscles, which they dilate and 

 contract at pleasure. It adheres like a sucker, 

 and the animal advances by fixing the fore part 

 to the ground and drawing the remainder after 

 it. This instrument is called a foot. In some 

 species, as the snail, it is attached to the under 

 part of the body by its whole length, but in 

 others it is free at one end, and can be extended 

 or drawn in at pleasure. When it is free it is 

 called a leg, and is generally tongue -shaped ; it 

 is frequently used as an organ of motion, but 

 sometimes the animal employs it as a paw for 

 digging holes in the sand or mud. But in what 

 situation do you think that this organ will not 

 enable the mollusks to advance ? 



Child. They could not use it in the water ; 

 there they must swim. 



Teacher. Yes, and this action is performed 

 either by the serpentine movement of the foot 

 and body, or by the movement of expanded por- 

 tions of the skin, or of their tentacula, forcing 

 them onwards in the same manner as oars pro- 

 pel a boat. 



Child. Have all the mollusks that live in 

 the water either tentacula or expansions of the 

 skin? 



Teacher. No ; some have no such elongated 



