The venous system is of very considerable magnitude. In his magnificent work, Poli* has given a very satisfactory account of its anatomy. It is more particularly remarkable in the Archidm, the Pinna, &c. It is destined to receiv-e the blood of the general circulation ; it is also destined to collect the whole of the fluids absorbed, and to direct these towards the bronchial apparatus, in which the blood with these added fluids undergoes a fresh elaboration. It is after having traversed the bronchial vessels (c, c,c, c, fig. 349, 351, j, fig. 348) that the blood revivified is carried to wards the auricle by the pulmonary veins, from whence it is sent to the ventricle, and by it forced anew to perform the round of the arterial cir culation.
The blood in the Conchiferous mollusks is colourless, or of a bluish white, very different from the hue it presents in the vertebrata ; it is but slightly viscid, and when it coagulates exhibits but a very small quantity of crassa mentum or solid matter.
Circulation then is an extremely simple function in the Conchiferous mollusks : an aortic ventricle gives the blood impulse enough to carry it through the two systems of vessels, to expel it from the heart and to bring it back again to the auricle. In other branchiferous animals, the auricle is sometimes adapted to give the blood a new impulse when it is about to pass through the branchire ; here, OD the.
contrary, the auricles do not receive the blood until it has been exposed to the revivifying influence of the organs of respiration.
Of the organs of respiration.— The whole of the Conchiferous mollusks respire by means of branchim (e, e,fig. 346). These or gans are variously disposed according to the form of the animal. They are symmetrical; and in almost all the genera there are two on each side. The branchim generally present the form of membranous leaflets, of a qua drangular shape, though often unequal. They are broad and short when the animal is glo bular, elongated and narrow when the animal is lengthened in its general form. In the greater number of genera the branchim are formed of two membranous layers or laminm (a, I), fig. 352) within the substance of which the branchial vessels descend with great regu larity. In several genera, as the Archidm and Pecten, the branchial vessels, instead of being connected parallel to one another within the thickness of a common membrane, continue unconnected through their entire length, and they are thus formed of a great number of extremely delicate filaments attached by the base within, a membranous pedicle, in which the branchial veins pursue their way towards the auricle. In a great many families and genera the branchite of one side have no com-. munication with those of the opposite side ; in some others however, as in the genus Unio, the four branchial laminx meet under the foot, and the whole of their vessels empty them selves into a. venous sinus of considerable
size.
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 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 branchiw, and this circum stance has led several anatomists to mistake these small shells for parasites. As in all the other animals having branchi, 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 laminT which form the walls of the foot. ln the siphoniferous 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 ovaryresting upon the centml 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 which is an egg capable of reproducing an individual similar to that from which it de rives its origin.