PTEROPODA.
This class was instituted by Cuvier, for the reception of a few genera, whose peculiar characters indicated the impropriety of suffering them to remain in any of those categories which had been previously established. All the species are small in size, and the attempts which have been hitherto made to investigate their internal structure have, in a great measure, failed in explaining the functions of the organs which are exhibited. The valuable papers of Cuvier, on the Clio, Pneumodermon, and Hyalea, include nearly all the accurate informa tion on the subject of which naturalists are in posses sion.
The general form of these animals is somewhat ovate. The tunic appears in some genera, as the Clio and Pneu modermon, to be double, the external one soft and thin, the internal exhibiting a fibrous structure, correspond ing to the muscular web of the skin of the higher classes. In these animals, however, these two layers are unconnected throughbut the greater part of their expansion. In some, as the cymbulia, the tunic is cartilaginous, while in others it is strengthened by a shell. In these last, the shell in the Limacina is a spiral univalve, covering the abdominal viscera, and in the Hyalca, where it serves the same purpose, it approaches in character a bivalve shell. It is, however, destitute of a hinge, the two valves being united together at their caudal margins, and there is no appearance of a trans verse adductor muscle.
The organs of motion in all the genera consist of two fins, or membranaceous expansions, seated one on each side the head. They have no foot wherewith to crawl, nor any suckers by which they can adhere to objects. They are therefore free animals, moving about in the water by means of their tins, and possessing, probably at the same time, a power of varying their specific gravity, as they are capable of varying, to a certain extent, the form of their bodies, and of enlarging or reducing their dimensions. There is nothing peculiar in their nervous system.
Their organs of digestion differ greatly from those of the cephalopods, which we have already considered.
They are generally regarded as destitute of eyes and ears. Their tentacula are either seated on the head, forming two complicated branches of filaments, or spread along the margin of the tunic. There arc no arms for seizing the food. The mouth, however, is furnished with lips, and, in some, an appearance of a tongue at the entrance of the gullet. The salivary glands are two in number, lengthened, descending a considerable way into the abdomen, and pouring their contents, by means of their excretory canals, into the ca vity of the mouth. gullet, after being encircled by the nsrvous collar, suffers an enlargement, which has been termed a crop, contiguous to which is the stomach. Both these cavities exhibit muscular ridges on the inner surface. The liver surrounds the stomach, and is inti mately united with its contents, and pours in its bile by numerous pores. The intestine is short, and after making one or two turns, ascends and terminates in the neck near the mouth.
The circulating system in this class has been but very imperfectly investigated. The pulmonic vessels are unknown, but systemic veins, a single auricle, ventricle, and aorta, have been detected. The heart, in some, is situated on the left, in others on the right side of the body.
The respiratory or aerating organs, exhibit very re markable differences. In the Clio they are in the form of a fine net work on the surface of the fins; in the Pneumodermon they are conjectured to form leaf-like ridges on the caudal extremity of the body, or if these ridges are to be considered as particular kinds of fins, the gills may be sought for on the membranaceous ex pansions of the neck. In the Hyalea the branchia: form a complex band on each side the body, at the lateral opening of the shell.
The animals of this class are all hermaphrodites. There is a common cavity, a vesicle, penis, vas deferens, and testicle, together with an oviduct and ovarium. These open near the mouth on its ventral margin. There is nothing known with respect to the appearance of the eggs, the period of propagating, or the form of their young.