I94 CONCHIFERA. 



in the first place, absolutely essential to the existence of these 

 animals that the element in immediate contact with the respi- 

 ratory surfaces should be renewed as rapidly as it becomes de- 

 teriorated, or suffocation would inevitably be the speedy result. 

 Secondly, it is natural to inquire, How is food conveyed into the 

 mouth ? for in an animal, itself fixed and motionless, quite de- 

 prived of any means of seizing prey, or even of protruding any 

 part of its body beyond the margin of its abode, it is not easy to 

 imagine by what procedure a due supply of nutriment is pro- 

 cured. 



FIG. 194. VENUS CHIONE. FIG. 195 SPINED VENUS. 



Wonderful, indeed, is the elaborate mechanism employed to 

 effect the double purpose of renewing the respired fluid, and 

 feeding the helpless inhabitant of these shells. Every filament 

 of the gill- fringe, examined under a powerful microscope, is found 

 to be covered with countless cilia, in constant vibration, causing, 

 by their united efforts, powerful and rapid currents, which, sweep- 

 ing over the entire surface of the gills, hurry towards the mouth 

 whatever floating animalcules or nutritious particles may be 

 brought within the limits of their action, and thus bring streams 

 of nutritive atoms to the very aperture through which they are 

 conveyed to the stomach, the lips and labial fringes acting as 

 sentinels to admit, or refuse entrance, as the matter supplied be 

 of a wholesome or pernicious character. So energetic, indeed, is 

 this ciliary movement over the entire extent of the branchial 

 organs, that if any portion of the gills be cut off with a pair of 

 scissors, it immediately swims away, and continues to row itself 

 in a given direction as long as the cilia upon its surface continue 

 their mysterious activity. 



