In the last group no pulmonic auricle or ventricle, neither systemic auricle, can be perceived ; but there is a single systemic ventricle. Animals of the class tuni cola exhibit this simple arrangement.
The circulating system furnishes few characters which can be employed in systematical arrangements. The structure of the systemic and pulmonary vessels does not appear to be co-ordinate with any particular plan of external configuration and manner, as we see in the case of the pteropoda and gasteropods. In these the organs of circulation are very much alike, while the external forms exhibit very obvious differences.
7. Respiratory is probable that all animals, however imperfect, stand in need of a supply of oxygen. In proportion as the circulating system becomes com plex, the greater is the quantity of oxygen which is re quired. In the more perfect animals it has been ascer tained, that this oxygen unites Stith carbon thrown out of the system, and, by converting it into carbonic acid, enables it more readily to escape in the gaseous form. In some animals this production of carbonic acid takes place generally throughout the surface of the skin, while in others particular organs are appropriated to its formation. This is more particularly the case in those animals which, like the molluscs, have pulmonic and systemic blood-vessels When the animal separates the oxygen from unmixed atmospheric air, the organ in which the process is performed is termed lung. When the separation takes place in atmospheric air united with water, the organ is termed a gill.
The molluscous animals which respire by means of lungs are few in number, and form a very natural tribe, which Cuvier has termed gasteropodes pulmones. In them the respiratory organ is simple, consisting of a single cavity, on the walls of which the extremities of the pulmonary artery are spread. This cavity is placed somewhat laterally, and communicates externally by an aperture which the animal can open or shut at pleasure.
Tlie mollusca which breathe by means of gills, exhibit very remarkable differences, in their number, structure, and position. In some cases there is a single cavity com municating by an aperture through which the water en ters. The walls of this cavity exhibit an uneven surface, disposed in ridges, which are the gills, and on which the pulmonic artery is expanded. This structure exhibits
itself in the Gasteropoda pectini-branchia.
In many cases the gills, though seated in a cavity, like the former, and equally exposed to the contact of the surrounding element, are two in number, one on each side, as in the Cephalopoda. In the Conchifera they arc four in number, two on each side, like leaves, and extend ing the whole length of the body. In these the water is admitted at the pleasure of the animal.
The gills of other mollusca are seated internally, and consist either of arboresccnt productions, or simple cu ticular elongations, within which the pulmonary artery terminates. In some of these, as the Pteropoda, the branchial surface is constantly exposed to the action of the surrounding water ; while in others, as the Gastero poda, nudibranchia, and tectibranchia, the cuticular ex pansions, which are analagous to gills, are retractile at the will of the animal.
By means of the characters furnished by the circulat ing and respiratory systems, the molluscous animals may be divided into several distinct classes. But as we shall employ these characters in the construction of the different divisions to be employed, it is unnecessary, in this place, to enter into their details.
8. Reproductive System.—The animals now under con sideration present nearly all the modifications of genera tion which organized bodies exhibit. They arc not in deed known to produce their young like buds, as in the fresh-water polypus (Hydra,) nor to multiply by the spon taneous division of individuals, as some Nereid and Planarim. But the three following etudes of reproduc tion have been detected.
In the first the sexes are distinct, as in the higher classes of animals. This mode is exhibited in the Ce phalopoda and some Gasteropoda. In the former, how ever, there is no union of the sexes previous to fecunda tion. The eggs are first excluded by the female, and the male afterwards covers them with the impregnating fluid, after the manner of fishes. In the Gasteropoda, with the sexes distinct, a union takes place by which the eggs are fecundated. After impregnation, the eggs are either excluded previous to the young issuing from them, or retained in the body of the animal until they are hatched.