was the first to perceive their utility in this respect; and when it is considered that their position mainly determines the arrangement of the other viscera, and must consequently exert a powerful influence over the habits of the animals, you will feel disposed to admit that a happier choice could not have been made, the more particularly as the organs in question are in general easy to detect, and exhibit sufficient variety in location and form for every systematic purpose.

Molluscous animals are either Pulmoniferous, and breathe atmospheric air only, or they are Branchiferous, and respire it through the medium of water. In the former, the respiratory organ is a simple cavity, commonly situated on the anterior part of the back; but sometimes, as in Testacélla, near the tail. The air is admitted by a small circular aperture that opens outwards on the neck under the margin of the cloak, and which the animal opens and shuts at pleasure. Externally the cavity is protected either by a thick fold of the cloak, often strengthened with a horny or calcareous plate, or by the body-whorl of the shell; while its interior walls, and more especially its floor, are covered with a fine vascular network, formed by the minute ramifications of the pulmonary vessels, which thus expose the blood freely to the influence of the air, alternately introduced and expelled by the alternate dilatation and contraction of the cavity itself. All the terrestrial Mollúsca, such as slugs and snails, and the great bulk of the Gasterópoda that inhabit fresh water, possess a respiratory apparatus of this kind; and, since these aquatic Pulmonífera (Lýmneus, Planórbis, and Ancýllus may be quoted as examples) are necessitated, from time to time, to inhale the fresh and uncombined air, so they will be found uniformly to be the denizens of shallow waters, and to spend a large portion of their lives at the surface.

The Branchiferous Mollúsca have the aerating organs greatly more diversified in every respect; and, to countervail the disadvantages of breathing a medium little impregnated with air, they are likewise of greater extent and complexity. When placed within the body, the branchiæ, if distinct, are divided into multiplied lobes and leaflets; or, if a mere cavity, the surfaces are folded into innumerable plaits, all calculated to afford ampler space for the display and meanderings of their blood-vessels, and to expose a wider surface to the contact of the water: but, if the branchiæ are external and exposed, they are, it may be, less complicated, only because complexity seems unnecessary where fresh doses of unbreathed fluid are continually brought into momentary

contact with them, and without any effort on the creature's part.

The Mollusca which have their branchiæ entirely exposed belong to two sub-classes, the Pterópoda and Gasteropoda. In some of these, the branchiæ are actually blended with the locomotive organs, as in Clio, a member of the former subclass, whose fin-like expansions are supposed to perform the office, not of progression only, but also of ventilating the blood as it circulates through the fine regular network with which their surfaces are covered. The Glaúcus (fig. 26.)

affords another example of the same union of functions. This is one of the most remarkable and most beautiful of the Gasteropoda. The body glows with a fine cærulean blue colour, which deepens in hue towards the ends of the fringes of its ptero-branchiæ; the centre of the back is of pearly whiteness, bordered with a line of deep blue; and the sides are adorned with an interrupted series of fanlike laciniated gills, by aid

of which, as I have said, it swims reversed at the surface of the Mediterranean Sea, in numerous swarms.

But, generally, the external branchia are distinct and independent organs. Of the Pteropoda, almost each genus

presents them under some new modification in form, or structure, or position, "as Nature in them strove to show variety." Thus, in the Pneumodérma (fig. 27.), they are placed nearly

27

on the posterior extremity of the body, which is naked, and resemble two Cs placed back to back in this manner, OC, united by a little transverse bar across the middle, or at each end, the lines being garnished with a number of regular prominent leaflets of minute size. In the Hyales (fig. 28.), again, the branchiæ are pectinated, and lie concealed in a space between Pneumodérma, the lobes of the cloak, to which the water gains copied from Cuvier. admission by certain fissures on the sides of the shell; while, to make, as it were the dissimilarity perfect, they appear, in the genus Cuvièra of Rang (fig. 29.), in the form of two small equal and symmetrical processes, exsertile

28

29

Hyale, copied from Rang.

Cuviera, copied from Rang.

beyond the shell when the animal is in motion, but at other times retracted, and fixed upon a common stalk by a point a little removed from one of the ends.*

The Gasteropoda which arrange themselves under this division form a very natural and interesting order, appropriately named by Cuvier the Nudibránchiæ. These are naked snail-like Mollúsca which live only in the sea; and they would scarcely attract our notice amid the myriads of curious creatures that are around them, were it not for the ornament and singularity of their branchial appendages. Their position is always on some part of the back, either ranged in one or more series along its margins, as in Glaucus, Eolidia, and the Tritonìada, or clustered on a point of the medial line near the hinder extremity, as in the Dòris and its allies, which have the power of concealing them within the body when danger threatens from without. In shape they vary more than in position: they are simple filaments in Eolídia; in Glaúcus they are fan-shaped fins; in Melibæ a clubbed processes, covered with little hispid tubercles, or, as in Scyllea, with little tufted bouquets of very delicate filaments; and in the Tritoniada and Dòris they assume a more or less perfectly plumose or arborescent appearance.

There are other Gasterópoda in which, although not so fully exposed as in the preceding order, the gills are still only slightly concealed by some lap or fold of the cloak. The genera Patélla*, as now restricted, and Chiton afford examples where the branchiæ, in form of a cord composed of pyramidal processes, or of close-set and parallel transverse

* Rang's Manuel de l'Histoire Naturelle des Mollusques, p. 38. tab. 2. fig. 4. An excellent work, which I regret I had not the assistance of in the compilation of the preceding letters.

† Blainville, however, maintains that Patélla is pulmoniferous. (Manuel, p. 125.)

leaflets, encircle the body more or less completely, lying in a furrow between the foot and cloak, and merely covered by the prominent margin of the latter. In an order which Cuvier calls Inferobránchiæ, these organs occupy a similar position, but limited often to one side of the body; while, in the Tectibránchiæ, an order of which Aplýsia may be selected as the type, the gills, almost free, and like some miniature arbuscle, occupy a position on the back, where they lie, scarcely hidden, under a movable corneous lid that sits in the centre of a hollow formed by the large and muscular dorsal fin, intended apparently to collect the water as in a crater, that it may not pass away too rapidly, and until it has thoroughly penetrated the intricacies of the branchial appa

ratus.

*

The Mollúsca with internal branchiæ are more numerous than the preceding; for some Pterópoda, the greater number of the Gasteropoda, all the Cephalopoda, and all the acephalous tribes are thus circumstanced. The various modifications of them in the remaining Gasterópoda I will not now dwell upon; for it will be necessary to describe them hereafter, when I explain to you the Cuvierian system: but there is one family which, because of its greatness, may not be passed over in this place, this is the Pectinibránchiæ; an order that includes almost all the marine turreted and convolute + shells, and a few which are found in fresh water. In it the branchial cavity has a position similar to that in the Pulmonífera, on the upper and fore part of the back; to wit, where it is protected by the body of the shell: but its walls are not smooth and even, like those of the pulmonary cavity, but folded into neat and regular plaits or ridges, that lie parallel to one another, like the teeth of a comb (whence the name Pectinibránchiæ), and often part on each side from a central stalk formed by the trunks of the blood-vessels, in the same manner that the barbs of the web of a quill depart from the shaft. The water obtains ingress to this cavity, in such Pectinibránchiæ as inhabit shells with entire apertures, by a large slit on the side above the collar; and, in shells with interrupted or beaked apertures, by an imperfect siphon that

* Lamarck considers the branchiæ of bivalves (Conchífera) as properly external (Hist. Nat., vol. v. p. 417.); and this view of their position is plausible, more particularly when the cloak is open in front.

Sir E. Home asserts that the lobes of the mantle which cover the shells of the cones and cowries are the respiratory organs of the animal (Comp. Anat., vol. i. p. 55.); but this is a mistake: they are true Pectinibranchiæ.

lies in the canal or emargination, and that is formed by a prolongation and duplicature of the cloak.

The gills of the Cephalopoda are placed, one on each side, within the sac, to which they are attached by a thick broad fleshy riband. Each gill is composed of two series of branchial leaves connected on one side to the fleshy riband just mentioned by short pedicles, and on the other uniting with the leaves of the opposite row; and, as the leaves of the one series are not opposed to, but alternate with, those of the other, each of them unites two leaves of its antagonist series; and they are in this manner all joined together at the margin farthest from the fleshy riband, and along which the branchial vein runs. Each leaf is itself garnished with little cross leaflets, and these are similarly divided (Cuvier, Mém. sur les Mollusques, vol. i. p. 20.); the structure, on the whole, reminding one of the mechanism of the gills in fishes.

The branchia of the bivalved Mollusca are always placed between the body and its cloak, the folds of which, being, in many of them, altogether separate in front, admit the circumfluent water very freely; but when these folds are soldered together at the edges, as they often are, the water is imbibed through a siphonal tube formed by an elongation of the cloak, and extruded at the posterior end of the shell; and the effete fluid is expelled again through the common excrementitious tube, which is altogether like the other, and occupies the same position. In the Branchiopoda, a small order of this class, the branchiæ are arranged in a pectinated form on the inner surface of each lobe of the cloak; but the other tribes have free gills in the form of large semilunar leaves that embrace the sides of the body. There is a pair of these, often of unequal size, on each side, and each leaf is joined to that which corresponds to it on the opposite side at the dorsal margin; but in front they are usually separate: they are broad and lamelliform, are finely and regularly striated across, and sometimes appear punctulated in the intervals of the striæ. * Each leaf, according to Blainville, is itself formed of two layers which leave between them a free space, divided by numerous triangular partitions into a great number of vertical cells open to the dorsal margin. These layers are constituted by two series of parallel vertical vessels united by others which cross them; one of the series

* These striæ appear to be formed of a small number of tubes, bound together. They may be compared to the nervures of the wings of insects; for they seem partly intended to keep the branchiæ even, and prevent them being rumpled. They are parallel to one another, and connected by cross tubes, that divide the interspaces into small regular squares.