the whales and other fishes, as well as to the sea-birds. This animal is abundant in places that suit it, and appears only during the warmest hours of the day on the surface.

Other genera of this Order are covered by a shell or shells. Of this kind is the genus Hyalaa, so named from its semitransparent shell, which wears the appearance of a bivalve with soldered valves, the upper one being the largest; this difference of size of the seeming valves causes an aperture through which the animal sends forth two large yellow and violet wings, or sails, rounded and divided at their summit into three lobes. The head in this genus is almost evanescent, so that both shell and head exhibit an easy transition from the acephalous or bivalve Molluscans to those which have a head. When its wings or sails are unfolded it moves with great velocity on the surface of the sea. The animals of this Order, both from the beautiful colouring of their filmy sails or wings, and from their number and symmetry, are better entitled to the appellation of the butterflies of the ocean, than the escallop shells which have sometimes been so called. The mantle of the bivalves becomes an organ of very different use in the Pteropods; for they, having no means of fixing themselves, like most of the bivalves, float continually in the ocean; to compensate for this want, as in innumerable other instances, their Creator has given them the power of expanding this organ as a sail, both for motion and to give some direction to their course; it is attached to the mouth or neck, and is connected in some species with their respiration. Nothing certain is known with respect to their food: probably they absorb the animalcules swarming in the

sea water.

2. The series of Gastropods begins with animals that have no shell, amongst which the most remarkable seem to be the Scyllea and the Tethys, both known to Linné, and by him described. The former is an oblong gelatinous animal, laterally compressed, elevated above in the middle, where it has two pair of membranous wings or fins. Its inferior surface is hollowed out longitudinally, by means of which, and its tentacles, it can embrace the stems of the fuci or sea-wrack, the flowers of which it eats. It is described as moving very slowly in the water by bending its extremities. It swims on the surface when the weather is calm, but adheres to the floating fuci when the sea is agitated, so that the kindness and foresight of its Maker-by giving it wings, for independent motion, and means to adhere to the fuci, when support is necessary to it, or it takes its food-has thus provided amply for its enjoyment and sustenance. The great peculiarity of the latter, the

Tethys, is a mantle which extends above and beyond the head, like that of some marine goddess, concealing it entirely, and forming an ample veil, fringed or undulated at its margin. By the help of this veil they elevate themselves to the surface, and probably sail on the waters. This animal is nearly related to the Laplysia, a kind of sea-slug, like which it lives in muddy places, and ejects a black fluid; it is very fetid, and its flesh is poisonous. It only rises to the surface in the hot season.

I shall next notice a tribe of Gastropods, which at first sight, considering the number of pieces of which their shelly covering is composed, seems to belong to the multivalves, amongst which Linné has placed it. It will be readily perceived that I am speaking of the Chiton, or coat-of-mail shell, but when the animal that it covers is examined, it will be found that, notwithstanding its multivalve shell, it really belongs to the Gastropods.

These animals are generally found under stones, sometimes they adhere to the surface of rocks, and sometimes conceal themselves in their fissures: they often traverse vast tracts of ocean fixed to the keels of ships: like some of the limpets, they fix themselves a good way out of the water, so as only to be wetted when the tide is up, and sometimes above high water mark. Poli says that when they resist any attempt to force them from their station, they expel the air and water on all sides and produce a vacuum, so that it is very difficult to overcome the pressure of the atmosphere; and Mr. Frembly, who had an opportunity of studying their habits on the coast of Chili, states that when not apprehensive of danger their attachment is very slight, and by pushing them gently they will easily slide from the surface to which they are attached, but if a direct attempt is made to unfix them by force, they will part with a portion of their shells sooner than let go their hold.

When we consider that these animals are not only often exposed to the violent action of the waves, but also to the attack of countless enemies, we see abundant reason for the coat of mail with which their Creator has covered them. Even the fleshy or cartilaginous margin, or zone, as my lamented friend the Rev. Lansdown Guilding, in his admirable memoir on this tribe, denominated it, is defended sometimes by scales, spines, and bristles, at others rough with numerous little bony tubercles; it is also described as in general fringed, so that when the animal attaches itself to a rock or stone, it is altogether calculated, by the application of the prone part of its body, to produce a vacuum. The wing-shell and other bivalves that suspend themselves by a byssus, are sufficiently protected by

their shells from the attack of their enemies, without so complete an adhesion of the body as is necessary for the coat-ofmail shell. Mr. Guilding, who had excellent opportunities of observation, informs as that these animals are night-feeders, remaining stationary as above, during the day; reasoning from analogy he suspects they feed on marine plants, the sea-wrack, &c. These creatures slide along very slowly: if accidentally reversed, they recover a prone position by the violent motions of the ligament or zone that surrounds them, and if alarmed they sometimes roll themselves up like woodlice.

Lamarck proceeds immediately from the Chitonidans to the Patellidans or Limpets,' which also fix themselves so firmly to the rock, that it requires considerable force to separate them, and sometimes in such numbers that their surface seems quite covered by them. The transition from the former tribe to this, with no intermediate links, seems at first sight violent, and their right to be associated in the same family rather problematical probably intermediate species will come to light which will render this point more evident than the shell of these animals appears to indicate.

With regard to their functions and the part assigned to them. in the great plan of creation, little is known; probably, from their numbers in some parts, they may help to soften the rocks, so that they may, at some destined hour, yield more readily to the force of the winds and waves; thus they may be enumerated amongst the instruments which the Creator employs to effect his purposes, and such changes in the coast of any country, as he wills shall take place.

They afford a beautiful instance of the gradual progress of Creative Wisdom from form to form. If the student of the tribe looks with inquiring eye at a collection of the Patellidans, or limpets, in the flattest and most depressed of them he will tind no small resemblance to one of the valves of a bivalve shell, he will soon, however, discover a prominence in it, the first tendency towards the spiral convolution, a little removed from its centre, which will prove to him that it belongs to a very different tribe; looking again at others that are more elevated and conical, he will see the same prominence or beak forming a more striking feature, and ascertaining these shells to be univalves, he will find, upon a comparison of them with the nerit, the snail, or the periwinkle, that this umbo or knob is analo

1 Patella.

3 Patella vulgata.

5 Beliz.

2 Umbrella indica.

4 Nerita, Neritina, &c. 6 Turbo.

5

gous to the spiral part of those shells, as he will see upon examining one of the bonnet-limpets,' in which he will detect an incipient decurved spire; passing from this by one of the chambered-limpets, it will lead him to the neritidans, or topshells, from which the road is direct to the sea-ear;3 and by another he arrives almost immediately at the periwinkles and snails. If he chance to examine farther between the limpets and the whelks, he will find another open shell," which forms the path to the latter genus. If once more his eye happens to observe a shell almost open, but with the sides a little turned in, he will see still another road leading by the dippers to the elegant tribe of cowries. It is by this road that Lamarck travels to them. Again, he may perhaps be shown, preserved in spirits, an animal whose respiratory orifice is covered by a round shield-this is the sea-slug, 10 an animal famous for Pliny's legend of its noxious qualities, whose head resembles a hare, which leads from the Patellidans towards the common slug of our gardens. To the bivalves there seems to be also a road from this central group, by a Norwegian shell described by Müller as an anomalous species of limpet, but which by Lamarck is considered to be a bivalve.12 The lower valve in this genus is so thin that Müller overlooked it; by it the animal adheres to marine bodies-the upper valve, like the Patella, is sub-conical with a prominent vertex, and the two valves are not connected by a hinge.

11

A due consideration of all these circumstances, of this radiation, as it were, from a typical form as a centre, by various roads towards different tribès, seems to prove, and the observation is confirmed by facts in other departments of nature, that the world of animals, as well as that of heavenly bodies, consists of numerous systems, each, so to speak, with its central orb, and all concatenated, and revolving as it were wheel within wheel, and all tending towards or branching from a common centre. It seems, in the present instance, taking the group expressed by Patella of Linné as the common centre, that from thence, though by different and diverging routes we may arrive at almost every molluscan group or tribe.

with the exception of the herbivorous chitons, derive their nutriment from the sea water itself, either from animalcules or other marine substances requiring only absorption, but the Gastropods that we are next to notice live upon more solid food, and such as cannot be digested without a more powerful action upon it. Of this description are the dippers, which are furnished with a singular organ or gizzard that proves their predaceous or carnivorous habits: the remaining genera are herbivorous, but as they exhibit no very interesting traits I shall proceed to the next Order.

The Trachelipods, constituting Lamarck's third Order of Molluscans, may be divided into those that are herbivorous, and those that are carnivorous, the first having no respiratory siphon, with which the others are furnished.

The herbivorous Trachelipods may be sub-divided into ter restrial and aquatic, and the latter into those that inhabit fresh water or salt. It is not known that any of the predaceous ones are terrestrial. The terrestrial ones not only devour the leaves and stems of plants, but some also attack their rods, one species, defended by an operculum or mouth-cover, devours those of the violet. Others of this tribe are found on trees, under moss, or feeding on the lichens; the shells of some of these are what are called turrited, or long and slender, with spiral whirls, resembling, in miniature, a lofty tower with a spiral staircase winding round it. By this attenuated structure their motions, in their close retreats, are less impeded. As it is in this tribe of univalves that the organ just mestioned, the operculum, or mouth-piece, first makes its appearance, it will not be improper here to give some account of it.

If we survey the various tribes of shel-bearing animals we find them defended from the injuries or attacks, to which their situation exposes them by various expedients, all of them indieating Power and Wisdom in their contrivance and formation, and Goodness in their end. These animals themselves all have a soft body furnished with organs of different kinds, suited to their station and purposes. Those that are below them in the scale, especially the naked Folypes, and gelatinous Radiaries, are still more frail and evanescent, but their organization is so inferior, that it is probably less subject to derangement from external accidents, or injuries are sooner remedied, than in that of the shell-fish-which, unless they were clad in some kind of mail, would probably soon perish. Accordingly we find 2 Cyclostoma elegans.

1 Bulla.

3 Clausilia