CILIATED BRANCHIAE. 469 



very rudimentary dimensions, expands in other spe- 

 cies to such a size as richly to merit the name of the 

 " foot," whereby it is generally designated, and which 

 we shall see, in a subsequent chapter, playing a very 

 important part in the O3conomy of these animals. 



Whoever for a moment reflects upon the arrange- 

 ment of the breathing apparatus in these bivalves, 

 and the position of their mouth, consisting, as it does, 

 of a simple orifice, unprovided with any prehensile 

 organs, must perceive that there are two circum- 

 stances connected with the oeconomy of a conchiferous 

 mollusk, and those not of secondary importance, by 

 no means easily accounted for. It is, in the first 

 place, absolutely essential to the existence of these 

 animals that the element in immediate contact with 

 the respiratory surfaces should be renewed as rapidly 

 as it becomes deteriorated, or suffocation would in- 

 evitably result from the inadequate supply of freshly 

 aerated water ; to secure which, especially when the 

 valves are shut, no adequate provision seems to exist. 

 Secondly, it is natural to inquire, how is food con- 

 veyed into the mouth ? for, in an animal, quite de- 

 prived of any means of seizing prey, or even of pro- 

 truding any part of its body beyond the margins of 

 its abode in search of provision, it is not easy to 

 imagine by what procedure a due supply of nutri- 

 ment is secured. Wonderful indeed is the elaborate 

 mechanism employed to effect the double purpose of 

 renewing the respired fluid and feeding the helpless 

 inhabitants of these shells ! Every filament of the 

 branchial fringe, when examined under a powerful 

 microscope, is found to be covered with countless 



