BRANCHIAL CURRENTS. 493 



retractile fleshy 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. 



(1292.) Whoever for a moment reflects upon the arrangement of the 

 branchial apparatus, and the position of the oral orifice, consisting, 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 

 inadequate supply of fresh and aerated water to secure which, espe- 

 cially when the valves of the shell are closed, no adequate provision 

 seems to exist. Secondly, it is natural to inquire, how is food con- 

 veyed 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, sweeping over the entire 

 surface of the gills, hurry towards the mouth whatever floating- animal- 

 cules or nutritious particles may be brought within the limits of their 

 action, and thus bring streams of nutritive molecules to the very aper- 

 ture 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 direc- 

 tion as long as the cilia upon its surface continue their mysterious 

 movements. 



(1293.) Our next investigations must be concerning the internal 

 anatomy of the CONCHIFEKOUS MOLLUSCA* In the Oyster, the general 

 disposition of the body resembles that of the Pecten described above ; 

 and the mouth, enclosed between two pairs of delicate lips, occupies a 

 similar position at the termination of the branchial lamellae. In this 

 well-known mollusk the oesophagus is extremely short, so that the 

 mouth appears to open at once into the stomachal cavity (fig. 248, a), 

 which is imbedded in the substance of the liver (d), the biliary 



