45 



times prolonged into two tubes, from one of whieh water is in- 

 haled, through the other ejected ; internal muscles adhering to the 

 shells, sometimes two to each valve, sometimes only one ; a mus- 

 cular organ or foot developed on the ventral side of the viscera 

 in some of the tivo-muscled kinds, and more or less exserted. 

 Shell ; always bivalve, the valves hinging together by the aid of 

 an elastic ligament, and by interlocking teeth. 



In descending the series of univalve mollusks through the classes of 

 Cephalopods, Gastropods, and Pteropods, Ave have advanced by tolerably 

 natural gradations, designating the respective groups by the peculiarities 

 of the organ of locomotion. In the bivalve mollusks the foot is at first 

 absent, or only rudimentary, and the animal is of widely different organi- 

 zation, and of widely different habits. Lamarck distinguished them as 

 a separate class of invertebrate animals, adopting the name Conchifera, to 

 signify that all are shell-bearing ; Cuvier introduced the negative appella- 

 tion of Acephafa, or headless, because they have either no head, or only a 

 very indistinct representative of one. De Blainville, and subsequently 

 Owen, selecting a character which is present, and at the same time pecu- 

 liar, throughout, the lamellar structure of the branchiae, designated the 

 class Lamellibranchiata. 



The Lamellibranchiate Mollusca are then, as any one about to partake 

 of an oyster may observe, a mass of viscera lying in the bed of a pair of 

 shells, furnished on each side with two elaborate gills in the form of vas- 

 cular lamellae, commonly known as the beard, and over these is a sac or 

 mantle, in two lobes, the calcifying organs of the animal, each secreting a 

 shell. The shells, or valves, are joined at the dorsal margin by interlock- 

 ing teeth, and by the aid of an elastic ligament, which has a tendency to 

 open the shell, in opposition to the action of a pair of internal muscles, or 

 of two pairs, by which the animal closes it. The Lamellibranchiates, or 

 Bivalves, as they are more commonly called, are all water animals, and 

 have for the most part a sort of tadpole existence in early life. All are 

 oviparous, and when escaped from the egg, commence life with a more or 

 less perfect metamorphosis. Many swim freely about by the help of vibra- 

 tile cilia, and are provided with eyes which become obsolete when no longer 

 needed. The mouth is simply an opening in the front part of the animal 

 leading to the stomach. It has none of the characteristics of the mouth 

 in the higher mollusks, but is nevertheless furnished with a pair of delicate 

 labial palps, which are supposed to be organs of taste. The fool, uidike 

 that of the higher mollusks, is narrow and keel-like, and is not so much 

 an organ of locomotion as an organ of general usefulness ; its chief em- 

 ployment is digging and burrowing in the sand. The mantle consists 

 sometimes of two disconnected lobes ; generally the lobes are united, 



