378 CONCHIFERA. 



organ (i), which, although in Pecten it exhibits very rudimentary 

 dimensions, expands in other species to such a size as richly to 

 merit the name of foot usually applied to it. 



(412.) Whoever for a moment reflects upon the arrangement of 

 the branchial apparatus, and the position of the oral orifice, consist- 

 ing, as it does, of a simple aperture unprovided with any prehensile 

 organs, must perceive that there are two circumstances connected 

 with the economy 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 inevitably be the speedy result of an in- 

 adequate supply of fresh and aerated water ; to secure which, 

 especially when the valves of the shell are closed, no adequate 

 provision seems to exist. Secondly, it is natural to enquire, 

 how is food conveyed into the mouth ? for in an animal, itself 

 fixed and motionless, and at the same time, as in the case of the 

 creature we are now considering, quite deprived of any means 

 of seizing prey, or even of protruding 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 nutriment is secured. 

 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 

 branchial 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 nu- 

 tritive molecules to the very aperture through which they are 

 conveyed into 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 the 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 movements. 



(413.) Our next investigations must be concerning the internal 



