18 LESSON II. 



ground, and drawing the remainder after it. This 

 organ is sometimes free, and can be extended 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 mollusks 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, eifect a change in their 

 position by ingenious contrivances ; thus, the 

 common scallops, by rapidly shutting the two 

 pieces of their shell, can transport themselves a 

 short distance ; and others send themselves for- 

 ward by drawing in water and ejecting it again 

 with great force. Many species are furnished 

 with a kind of air-bladder, by inflating or con- 

 tracting which, they rise and sink in the water 

 as circumstances 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 afirm 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. 



