There is a singular genus place.d by the generality of writers in alliance with the Oyster, and designated by the name of Anomia, in which the ovary forms no part of the common mass of the viscera, but extends tketween the two walls of the mantle, which it separates in proportion as it increases in size. This position of the ovary in the substance of the skin is analogous to what is observed in the Terebratulx, in which the ovary is divided into four segments comprised within the substance of the mantle and in the direction of the prin cipal branchial vessels.
Notwithstanding the minute dissections which have been made of the acephatous mollusks, there are a great many in which the oviduct remains unknown. In two of these animals in which it has been sought for in vain, it has yet been seen running to wards the middle and anterior part of the branchiw, and opening to the right between the folds of this side. It is not yet known whether or not it be by this opening that the ova escape after they have undergone incubation in the branchiw, or whether they escape by the edges of these organs.
M. Prevost of Geneva has made some important observations on the generation of the Uniones, which appear to prove that although coitus cannot take place between the acephala, it is nevertheless necessary to their propagation that a certain number of these animals be found together near the same spot. From these experiments we may infer that a fecundating fluid is diffused in the water and absorbed by the ovary, which is thus fecun dated without the contact of two individuals. This phenomenon is comparable to that which we know takes place in the fecundation of the ova of fishes ; these are deposited by the female, and afterwards sprinkled by the male, who places himself above them, with the prolific fluid. Before adopting definitively the results of M. Prevost's experiments, however, it were necessary to repeat them a great number of times, in order to leave no doubts on this ques tion, so interesting to the naturalist as well as to the physiologist, touching the generation of the hermaphrodite mollusca.
The number of eggs extruded by each in dividual is very great, and explains the rapidity with which these animals are propagated in certain seas, and the production by accumulated generations of those extensive beds of shells which are so frequently found covering the sur face of actually existing continents.
Oro.ans of motion.—The °mans of motion
are of two kinds : one is destined to move the two valves with which the animal is covered; the other is peculiar to a special organ, by means of which the animal moves its whole body. The muscles may therefore be arranged into two classes : 1st, adductor muscles of the valves ; 2d, locomotory muscles, or muscles proper to certain organs. Those fleshy and fibrous fasciculi attached between the tsvo shells, and which by their contraction approxi mate and close these two shells, are denomina ted the adductor muscles. In the greater num ber of the conchiferous mollusca, two of these muscles can be demonstrated, the one anterior (rig. 346 ; h, fig. 347; a, fig. 362) situated in front of the oral aperture, and the other pos terior (f, jig. 346 ; fig. 347; b, fig. 362).
Lamarck has given the title of Dimyaires to all the mollusca having two adductor muscles, a character which he has invested with a consi derable degree of importance, because it is con stantly proclaimed by the interiors of shells, upon which the impression left by these mus cles is very distinctly seen (a,b,fig. 367). One of these muscles, the anterior, diminishes gra dually as we descend in the series of the Con chifera ; in the family of Mytilacea it only exists in a rudimentary state (a,fig. 353); and after these it disappears entirely. In propor tion as the anterior muscle disappears, the pos terior one increases in size, and approaches more nearly to the middle of the valves. When DO farther trace of anterior muscle can be dis covered, the posterior muScle continues singly (k,fig. 348), and the mollusca having a single muscle, very distinct from the former which have two, have received the name of Mono myaires from M. Lamarck.
Poli, however, has shewn that the muscle of the Monomyaria consists in reality of two por tions, readily separable from one another, and even differing considerably in their appearance. This leads us to presume, with every show of reason, that the single muscle in the Mono myaria is the result of the approximation of the two muscles, which are parted in the Dimyaria. This fact would incline us to regard the num ber of the muscles as a matter of but small im portance in the classification of the conehiferous mollusks, and we may suppose that it was with such inductions before him that Cuvier was led to attach such slight significance to the division of these animals proposed by La marck.