MOLLUSCA. 167 



branous plates; and, as the Latin -word lamella means a plate, 

 the compound term above employed denotes that structural 

 peculiarity by which the class is distiuj^uished. It includes 

 the oyster, the scallop, the cockle, the mussel, and other well- 

 known bivalves. 



The sexes are distinct. The ova remain, for some time, in 

 receptacles within the gills, which ai'c thus made to perform 

 the office of a marsupial sac; and here the young of some 

 species, in their more advanced state, may be observed swim- 

 ming freely about. The young of others anchor themselves, 

 after exclusion from the parent, by means of silken filaments 

 which arc wanting in the mature individual, thus furnishing 

 to the naturalist a beautiful example of " prospective design 

 for the well-being of the weak and defenceless." * 



The mouth of the oyster is situated near the hinge, beneath 

 a kind of hood formed by the edges of the mantle {Fig. 

 159). But the question naturally arises, how is it supplied 

 with food, the animal itself being utterly incapable of any 

 active exertion for that purpose? We shall answer in tho 

 words of Professor Rymer Jones: — "Wonderful, indeed, is 

 the elaborate mechanism employed to effect the double purpose 

 of renewing the respired fluid and feeding the helpless in- 

 habitants 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, huny towards 

 the mouth whatever floating animalcules or nutritious particles 

 may be brought Avithin the limits of their action, and thus 

 bring streams of nutritive molecules to the very aperture 

 through which they are conveyed to the stomach, the lips and 

 labial fringes acting as scnlinels to admit or refuse entrance, 

 as the matter supplied may be of a wholesome or pernicious 

 character."! Furnished with an apparatus so effectual, we 

 can imagine that these animals reahse the condition described 

 by the poet; and, 



"In their pearly shells at ease, attend 



Moist nourishment." — SIiltox. 



If, however, while the oysters are thus lying " at ease," tho 



* Owen, pages 289, 290. 



f Outline of the Animal Kingdom, page 378. 



