MOLLUSCA. 19 



Child. They have not any limbs. 



Teacher. Very true. They have not limbs 

 or joints, as insects, nor are they divided, or 

 moved by means of rings, as worms. 



Child. Yet they move about. 



Teacher. Yes ; the greater part of the mol- 

 lusca have the power of locomotion ; that is, the 

 power of moving from place to place ; it is de- 

 rived from Latin locus, a place. We will first 

 examine the organs by means of which it is ex- 

 ercised. In what manner do snails and slugs 

 make their way along the ground ? 



Child. Their skin seems to expand at the 

 side of the body, and to adhere to the earth, and 

 then they draw themselves on. 



Teacher. This fleshy expansion under their 

 bodies 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 

 withdrawn at pleasure. When it is free it is 

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

 frequently used as an organ of motion, but some- 

 times the animal employs it as a paw for digging 

 holes in the sand or mud. But consider whether 

 this organ will enable the mollusca to advance in 

 all the situations in which they are placed ? 



Child. No, not when they are in the we.ter ; 

 then they must swim. 



