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cous secretion, and again lines it as before. The mucous secretion first deposited forms the outer coat of the shell, and is of use in protecting it from injury, while the mucous matter mixed with lime, which is placed within it, forms the substance of the shell itself This deposition of mucus, and of mucus mixed with calcareous matter, goes on as the animal grows and feels the want of a larger shell for its protection: the shell is, in fact, moulded on the body of the animal itself, as the body grows; and for this reason any irregularity in the body is moulded in the shell.

The animal has the faculty, also, of mending any break or injury that its shell may have received, if it is not of such a magnitude as to derange all the functions of the animal itself; and it mends them in the same manner as it forms its shell; that is to say, by depositing first a coat of animal matter, and then lining it with mucous matter, mixed with chalk, to harden it. But as the animal is usually very desirous of getting the repairs done as quickly as possible, and is most probably damaged by the injury it has received, these repairs are generally much more roughly executed than the shell itself, and commonly destitute of regular colour.

The particles which vary the colour of the surface of the shell are deposited while the shell is being increased in size, immediately under the outer mucous coat; and as these particles are also secreted by peculiar glands, the colour is always situated in a particular manner on each species, the glands being gradually enlarged, and gradually separated, but not changed in position by the growth of the shell.

All the variations exhibited in the colouring of the different species, or in the different individuals of the same species, are produced by the permanent or temporary interruption of the action of these glands. But for a more detailed account of these phenomena, I must refer the reader to my papers on the subject in the Philosophical Transactions for 1833.

OF CRAWLING MOLLUSCANS.

CLASS I. GASTEROPODA Cuv.

THE animal is furnished with a distinct head, two or four tentacles, and a broad expanded foot for locomotion; and is generally protected by a straight, oblique, or spirally-twisted conical univalve shell. This class is divided into orders, according to the form of their respiratory organs; thus:

A. The gills comb-like, formed of a ridge of plates or filaments, on the inner side of the mantle, over the back of the neck. (Ptenobranchiata.)

I. Zoophaga, the edge of the mantle produced into a siphon.

II. Phytophaga, the edge of the mantle simple. B. The gills variable, arborescent, or the respiratory organs in the form of lungs.

III. Pleurobranchiata, the gills on the side of the body, under the edge of the mantle.

IV. Gymnobranchiata, the gills naked on the back, or round the inner edge of the mantle.

V. Pneumobranchiata, the respiratory organs consisting of a bag formed by the mantle, and lined with the pulmonary vessels,

The last order consists almost entirely of terrestrial or fluviatile Mollusca, their organisation being only adapted for respiring free air; and there are a few fluviatile species found in the second order: the rest are all marine, and therefore excluded from our consideration at present.

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THE gills are in the form of one or more comb-like ridges of plates or filaments on the inner side of the mantle over the back of the neck. The edge of the mantle is entire, and destitute of any syphon. They respire water, or more properly air, through the water, and they are unisexual, and have only two tentacles; their mouth is usually at the end of a short proboscis, and they live chiefly on vegetable food.

The shells have an entire roundish or semilunar

mouth, without any canal in front. They are provided with an operculum, which covers the mouth of the shell upon the animal, and which, from its position and the manner of its formation being similar to that of the shell, may be considered as a free second en valve.

This order is divided into groups, by the position of the eyes, and the absence or presence of tentacles on the side of the body.

Sect. I. Podophthalma

Have the eyes placed on a separate pedicel at the hinder edge of the tentacles: the heart is generally traversed by the rectum, as in the Conchifera.

Fam. 1. Neritida. The sides of the body simple: shell ovate; mouth half-ovate, with an acute inner lip; operculum appendaged. (f. 1—3.)

Sect. II. Eriophthalma,

Eyes of the animal sessile at the base of the tentacles: diœcious.

Fam. 2. Melaniada, - animal; trunk elongate: shell

ovate; mouth ovate, not continued; operculum horny, ovate, spiral. (f. 4—6.)

Fam. 3. Paludinide, - animal; gills enclosed: shell conical; mouth ovate, continued; operculum annular. (f. 7—11.)

Fam. 4. Valvatida,-animal; gills exserted: shell conical; mouth round, continued; operculum horny, orbicular, many-whorled. (f. 12-14.)

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