18 LESSON II. 



the ground, and drawing the remainder after it. 

 This organ is sometimes free, and can be ex- 

 tended or contracted at pleasure ; it is then 

 called a leg, and is used either as an organ of 

 motion, or as a paw for digging holes in the 

 sand or mud. In the water, some mollusca 

 advance by means of the serpentine motion 

 of their bodies, others by the movement of either 

 expanded portion of the skin or tentacula. 

 Some, quite destitute of any separate organs of 

 motion, effect a change in their position by 

 ingenious contrivances ; thus, the common scal- 

 lops, by rapidly shutting the two pieces of 

 their shell, can transport themselves a short 

 distance ; and others send themselves forward 

 by drawing in water and ejecting it again with 

 great force. Many species are furnished with a 

 kind of air bladder, by inflating or contracting 

 which, they rise and sink in the water as cir- 

 cumstances may require. Some, however, have 

 no power at all of moving, but remain fixed 

 through life to the spot where they commenced 

 their existence. Their modes of attachment 

 vary ; some firmly fix themselves by the same 

 materials of which they make their shells : 

 others glue themselves by a viscid cement drawn 

 from glands in their bodies ; and, others throw 

 out a byssus, and anchor themselves securely to 

 some rock. The limpet, by forming a vacuum 

 in his shell maintains a firm hold of marine sub- 

 stances. The shells thus rendered stationary, 

 are called fixed shells, whilst those inhabited by 

 animals that move about, are termed free shells. 



