204 



GASTEROPODA. 



or retracted into elegant spiral folds at the pleasure of the animal. 

 The most obvious function attributable to the tentacular arms 

 is that of procuring food ; for, being otherwise deprived of pre- 

 hensile instruments, without some adequate mechanism these 

 helpless creatures, imprisoned in their shelly covering and fixed 

 in one locality, would be utterly unable to obtain the nourishment 

 necessary for their support. The contrivance for this purpose is 



found in the arms, which, covered 

 by cilia, produce powerful cur- 

 rents in the surrounding water; 

 and these, being directed towards 

 the mouth as to a focus, hurry 

 down the throat of the animal 

 whatever nutritive particles may 

 happen to be in the neighbour- 

 hood. The muscles supplied for 

 closing the shell in the Conchifera 

 are never more than two in num- 

 ber, and these pass immediately 

 from one valve to the other : in 

 the Brachiopods, on the contrary, 



FIG. 309.-SHELLV FRAMEWORK OF BRACHIOPOD. the HlUSCular System IS VCiy COm- 



plicated, no fewer than six pairs 



being provided either to act upon the valves or to move the animal 

 upon its pedicle. Their shells, moreover, contain a complex frame- 

 work for the support of the arms (Fig. 209). Seeing, therefore, 

 that these creatures differ from all other bivalves in almost every 

 part of their structure, there can be little doubt of the propriety 

 of considering them as forming a distinct class. 



CHAPTER XIX. 



FIFTH CLASS OF MOLLUSKS, 



GASTEROPODA.* 



THE Gasteropod Mollusks are so named on account of the 

 . peculiarity of their locomotive apparatus. The inferior 

 surface of the body is spread out into a broad fleshy disk or foot, 

 on which the animal crawls with an uniform gliding motion. The 



* yaaT-fjp, gaster, the belly; TTOVS, irodbs, pous, podos, afoot. 



