MOLLUSCA. 25 



fore part to the ground, and drawing the re- 

 mainder 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, the mollusca 

 advance either by means of the serpentine move- 

 ment of their bodies, or by the movement of 

 either expanded portion of the skin or tenta- 

 cula. Some, quite destitute of any separate 

 organs of motion, effect a change in their po- 

 sition by ingenious contrivances ; thus, the com- 

 mon scallops, by rapidly shutting the two pieces 

 of their shell, can transport themselves a short 

 distance ; and others propel themselves by draw- 

 ing in water arid 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 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, main- 

 tains a firm hold of marine substances. The 

 shells thus rendered stationary, are called fixed 

 shells, whilst those inhabited by animals that 

 move about, are termed free shells. 

 3 



