arched, they would have been less capable of resisting pressure if they had not been furnished with ribs and bosses, variously disposed in the different species, for the purpose of strengthening them. In the species represented in the accompanying figure, these ribs are very prominent.

898.

Several kinds of chambered shells, with sinuous partitions, are met with in various strata. These seem to have borne the same relation to the Ammonite, as the Orthoceratite to the Nautilus; and they have been arranged, according to their minuter diversities of structure. Some of them are spirally

curved, but the several whorls or turns of the spire are not in contact with each other; this is the case with the Crioceratite. Others are straight, or but slightly curved; such as the Baculite. The Turrilite, again, has more of a corkscrew curve, resembling that of many Gasteropod shells.-There is good reason to believe that this group, which most abounded at the period when the Ichthyosaurus and Plesiosaurus were the principal inhabitants of the sea and shores, was preyed on by those marine tyrants; the remains of the beaks, and even of the horrry rings surrounding the suckers, of Cephalopods, having been found in the fossilized excrement of those Reptiles.

CHAPTER XVI.

OF THE CLASSES OF PTEROPODA AND HETEROPODA.

899. NOTWITHSTANDING the small size of the class PTEROPODA, it is interesting in many particulars. It may be regarded as representing, in the sub-kingdom Mollusca, the Birds of the Vertebrated sub-kingdoms, and the Insects of the Articulated series; and the inconsiderable number of distinct forms which it presents, may probably be accounted for by reverting to the wide departure from the usual Molluscous type, which the animals of this class exhibit in their structure and habits. They are particularly distinguished by the possession of a pair of fin-like

organs, or wings, consisting of an expansion of the mantle on each side of the neck, and furnished with muscular fibres; by the aid of which instruments, they can be rapidly propelled through the water. The body is uniformly symmetrical; that is, its two sides precisely correspond,-a condition evidently favourable for rapid movement. It is from the wing-like character of these lateral appendages, that the name of the class, which means wing-footed, is derived. Although the number of species belonging to this Class is small, and their dimensions are

inconsiderable, yet the number of individuals which associate together in shoals is often enormous, so that the sea appears literally alive with them. Some of them are possessed of a shell; whilst others are unprovided with such a protection. Where it exists, it is very light and delicate; and it seldom covers more than the posterior half of the body (Fig. 552). In one beautiful little Mollusk, the Cymbulia, it has the form of a slipper; from the large opening of which, the wings or fins are put forth. The head of these animals is usually prominent, possessing eyes and sensory tentacula; and their internal organisation is of a very complex nature.

FIG. 552.-HYALEA.

900. The Clio (Fig. 551) is one of the best-known genera of this class; and its general aspect conveys a good idea of that of the whole group. One species of this genus, the Clio borealis, abounds in the Arctic seas; presenting itself in such vast numbers, that, when the weather is calm, the surface appears covered with them for miles together; and an analogous species, the Clio australis, appears to be equally abundant in the polar regions of the southern hemisphere. These animals are well known to the whale-fishers and others, as whales' food, being among the chief articles on which that monster,-the largest existing animal,-is supported. It has been asserted that the sea is sometimes so glutted with the Clios, that the Whale cannot open its mouth without ingulphing thousands of them.-The chief point of special interest in the structure of the Clio, is the conformation of its organs for the capture and mastication of its food. The six tentacula, which are seen to project from the head, and which appear at first sight to be merely fleshy appendages, are in reality instruments of prehension, most elaborately constructed. Each of these six appendages, when examined attentively, is seen to be of a reddish tint; and this colour, under the microscope, is found to be dependent upon the presence of numerous minute isolated red points, distributed over its surface.

CLIO BOREALIS.-HETEROPODA.

359

When still further magnified, these distinct points are evidently peculiar organs, arranged with great regularity, so as to give a speckled appearance to the whole of the conical appendage; and their number, at a rough guess, may be estimated to be about 3000 on each. When very minutely examined, every one of these specks is seen to consist of a transparent cylinder, not unlike the cell of a polype, and containing within its cavity about twenty sucking-discs, mounted upon stalks, by which they can be made to project beyond the edge of their sheath, so as to apply themselves to their prey. Thus, therefore, the head of one Clio will bear about (3000 × 20 × 6) 360,000 of these microscopic suckers; an apparatus for prehension, which is, perhaps, unparalleled in the whole animal kingdom. In this manner, these active little animals are enabled to seize and hold their minute prey; and their mouths are furnished with efficient instruments for reducing it. The jaws, which are placed laterally, as in the Articulata, are furnished with long sharp comb-like projections or teeth; and the tongue is beset with a vast number of sharp spiny hooklets, curved backwards. Besides the prehensile appendages just mentioned, two sensory tentacula are capable of being put forth, for the purpose of feeling for the food. The Clio possesses eyes, which, though extremely minute, have a very complete organisation; and altogether its structure completely corresponds with what has been already remarked in regard to the character of the Class, as the Molluscous representative of Birds and Insects.

OF THE CLASS OF HETEROPODA.

901. The Mollusks of this class (which has been generally considered as an Order of the Gasteropoda, but of which late researches appear to require the removal to its present position) are distinguished from all others by the structure and position of the foot. This, instead of forming a horizontal disc more or less flattened, is compressed, so as to constitute a vertical mus

360

HETEROPODA.-CARINARIA.

cular paddle, serving as a fin. The edge of this, in many species, is somewhat dilated at one part; forming a kind of sucker, by which the animals can attach themselves, and which seems to be a rudiment of the expanded foot of the Gasteropods. The gills are external, and form plume-like tufts, situated at the hinder part of the back. The body is gelatinous in its consistence, and so transparent as to permit much of its interior organisation to be seen. The mouth is furnished with a sort of muscular tube or proboscis, which can be protruded or drawn in at will; and it contains a rasp-like tongue.-One of the most remarkable

FIG. 553.-CARINARIA; a, mouth; b, tentacula; c, eyes; d, stomach; e, gills; f, anus; g, shell; h, liver; i, foot; k, sucker.

genera of this group, is the Carinario; which possesses a thin shell, not unlike that of the Argonaut in form (§ 890). This shell protects the heart and liver; the gills float around its edge, and the intestinal canal terminates in its interior.-All the known animals of this group are inhabitants of the warmer-temperate, and tropical seas; many of them are found in the Mediterranean. Their general form and structure correspond very closely with those of the Carinaria.