700 



CONCHIFERA. 



A remarkable phenomenon is observed in a 

 great many of the Conchiferous mollusks : the 

 eggs on escaping from the ovary, instead of 

 being cast out altogether, are deposited between 

 the two membranes of the branchial laminae, 

 and there undergo a kind of incubation, 

 during which they acquire a considerable size. 

 In some genera, such as the Unio, the shell 

 is even des eloped within the egg before this is 

 cast loose from the branchiae, and this circum- 

 stance has led several anatomists to mistake 

 these small shells for parasites. As in all the 

 other animals having branchiae, the organs of 

 respiration are destined to restore to the blood 

 the oxygen which it had lost in its circulation 

 through the body. This necessary element to 

 the maintenance of life is restored to it during 

 its passage through an organ contrived so as to 

 bring it almost into contact with the ambient 

 fluid in which a considerable quantity of atmo- 

 spheric air, and consequently of oxygen, is 

 found dissolved. 



Organs of generation. The organs of ge- 

 neration are of extreme simplicity in the Con- 

 chiferous mollusks. They consist of an ovary 

 included in the visceral mass. Not a trace of 

 any other organ of generation can be detected, 

 and the Conchifera must therefore be allowed to 

 possess what has been called sufficient herma- 

 phrodism, generation in them taking place 

 without coition. The ovary is a glandular 

 mass situated at the superior and posterior part 

 of the body; it is in connexion with the liver ; 

 and it often receives a portion of the intestine, 

 if it happens to be developed laterally between 

 the two fleshy laminae which form the walls of 

 the foot. In thesiphoniferous acephala having 

 the foot short and rudimentary, the ovary, in 

 its state of complete development, forms a very 

 great part of the abdominal mass, amid which 

 it is easily distinguished by its soft consistency 

 and yellowish white colour. In those acephala 

 in which the siphon is short and the foot well 

 developed, the ovary forms a mass less promi- 

 nent at the superior and posterior parts of the 

 viscera. In the Conchifera monomyaria the 

 ovary resting upon the central muscle is situated 

 in the upper and posterior part of the body, 

 and in its state of development constitutes a 

 whitish mass of considerable size, which is 

 readily seen in the Ostracea through the walls 

 of the mantle. This ovary occupies the whole 

 superior part of the animal, and it is seen de- 

 scending along the lateral and posterior parts 

 when the animal is examined at the time of 

 laying its eggs ; a rent in the ovary allows a 

 fluid of a milk-white colour to escape. This 

 fluid under the microscope is seen to contain a 

 very great number of small whitish granules, 

 each of whkh is an egg capable of reproducing 

 an individual similar to that from which it de- 

 rives its origin. 



There is a singular genus placed 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 

 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 

 Terebratulae, 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 acephalous 

 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 

 branchiae, 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 

 branchiae, 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. Pre vest'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. 



Organs of motion. The organs 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 two 

 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 

 (f,fg. 346; h,fg. 347; a, fig. 362) situated 

 in front of the oral aperture, and the other pos- 

 terior (f, fg. 346; /, fig. 347; b, fig. 362). 



