Reproduction of lost parts.—Not less won derful is the power which snails possess of repro ducing lost parts, after mutilation by accident or design. The results of the experiments of Spallanzani upon this subject are very curious ; he found that if the large tentacle of a snail were amputated, the extremity of the stump heals, forming a small swelling of a lighter colour than the rest of the horn; in this swel ling a black point soon becomes visible, which is a new eye, and the mutilated member, in creasing in length, shortly equals its original size, although it is for some time of a lighter colour than its uninjured fellow, which in other respects it perfectly resembles. The process sometimes varies a little; it frequently happens that the end of the stump, instead of becom ing round, is elongated and tapers to a point, from the apex of which the new eye is seen to " squeeze out ;" the end of the tentacle then assumes a globular shape, and the most accu rate dissection cannot distinguish the newly formed eye from the original. If, instead of the horn, the head is cut quite off, a new one will succeed : the new head, however, does not at first contain all the parts of the old one, but they are gradually developed, piece by piece, at different intervals, until at length a head differing little, if at all, from the original pattern is completed. In some cases the ob ject is effected by a different proceeding, the new part appearing like a round tubercle, con taining the rudiments of the lips and of the smaller horns, which is united to the mouth and the new-formed tooth, the other parts, as the larger horns and the anterior part of the foot, being totally deficient. In another snail the larger tentacle on the right side first ap peared, not more than one-tenth of an inch in length, but already provided with its eye, and at a short distance beneath this the linea ments of the lips separately developed them selves. In a third snail a group of three horns is seen, two of which will acquire their full developement, while the third is just above the level of the skin. These and many other varieties have been observed ; but in most instances there is no perceptible difference between the new head and the one cut off; the exact line of separation being indicated by an ash-coloured mark distinguishable two years after the experiment. The same effects follow, whether the head be removed above or below the brain, and in the latter case a new brain, with all its nerves, is speedily con structed. The collar and foot are also per fectly restored after their removal.
Slugs reproduce their horns as well as snails, but their power of manufacturing a new head is much inferior.
Muscular integument.—None of the Gastc ropoda have any thing analogous to an endo skeleton, a circumstance which sufficiently ac counts for the varied forms which the same in dividual assumes under different circumstances, for the body being unsupported by any re sisting framework, readily yields to the con tractions of the muscular integument with which it is covered. It is from this circum stance that the zoologist finds the preservation of the natural forms of the recent animals a task of such extreme difficulty, owing to the corrugation and distortion produced by the or dinary modes of preservation; it is scarcely possible indeed, in many cases, to recognise with tolerable accuracy the natural appearance of these creatures in the shrunken specimens generally preserved in our cabinets, and the collector of these objects would do well never to omit, when circumstances allow him the op portunity, to preserve some sketch of the living forms of such exotic species as may come into his possession.
Body.—In the naked Gasteropods the whole body is found to be inclosed in a muscular in tegument, the basis of which is a cellular web of extraordinarily extensible character, in which the muscular fibres may be seen to cross each other in various directions, some passing longi tudinally from one extremity of the animal to wards the opposite end, while others, assuming different degrees of obliquity, are interwoven with the rest, so as to occasion the elongation or contraction of the body in every assignable direction. Within this muscular bag the vis cera are contained, as well as the organs sub servient to mastication, the apparatus of the external senses, and of the organs employed in copulation, which are, when unemployed, re tracted within its cavity by special muscular fasciculi spoken of elsewhere.
Rt tractile muscics.—In the spirivalve genera the muscular walls which inclose the body only exist in such parts as, during the extended state of the animal, are protruded from the shell ; that part of the body which is concealed within its cavity being provided with a much more delicate and membranous envelope ; in such, however, a necessity exists for an additional muscular apparatus, serving to retract the body and foot within the cavity of its calcareous abode, and of course exhibiting various modifi cations of arrangement in conformity with the shape of the shell itself. In the turbinated shells, the retracting muscles consist of strong fasciculi of fibres arising from the columella or axis of the shell, and diverging from this point, spread in several slips, which become interlaced with the fibres composing the foot and muscular investment. In the flattened forms of I'atella and Chiton, the muscular fibres arise all around the margin of the shell, excepting at its anterior part; these penetrating the mantle are intimately interwoven with the muscles forming the circumference of the foot. The animal of the Ilaliotis is fixed to its ex panded and semi-turbinated shell by a single large and ovoid muscle, which takes its origin from near the middle of the last spire ; what ever the disposition of these muscles, however, their action is obviously of two kinds ; and not only are they the agents by which the creature retires within its covering, but by raising the central portion of the disc of the foot, whilst its margins are in apposition with the plane of progression, they will, by producing a vacuum beneath, convert the whole apparatus into a sucker, the adhesive power of which will be proportioned to the extent of its surface.
Foot.—The foot of the Gasteropoda is their principal agent of progression. It is generally a fleshy disc, of variable size and shape, attached to the ventral surface, and forming when ex panded an organ by means of which the animal can adhere to surrounding objects. In the naked genera it is small, but in the conchife rous species, especially in such as are provided with dense and weighty shells, its dimensions and force are proportionally increased. In its internal structure it resembles the muscular in vestment of the body, of which in fact it is merely an expansion, consisting of muscular fibres interlacing each other in every possible direction, as may be developed by continued maceration. In the Slug, when opened from the back, the superior layer of fibres is found to run transversely, arising apparently from two tendinous lines which run longitudinally near the centre of the organ, and terminating near the margins of the disc ; beneath these, longi tudinal fasciculi may be detected, but so inter laced with other fibres assuming every degree of obliquity, that it is impossible to unravel the complicated structure which they form. In the Limpet (Patella) the lower fibres of the foot are transverse, hut near the circumference they become distinctly interwoven with circular fas ciculi ; the superior stratum viewed from above consists of two series of oblique fibres, which meet at an acute angle on the middle line, whilst the substance of the organ is composed of muscular bands variously disposed : from such a structure the movements of the foot are readily understood ; the transverse fibres by their contraction will elongate the ellipsis of the foot by diminishing its breadth, whilst the longitudinal, having a contrary action, will, by the combination of their effects, produce every movement needful for the progression of the creature. On minutely inspecting the foot of a terrestrial Gasteropod, as it crawls upon a transparent surface, it will be found to be divided into a certain number of transverse seg ments of variable size by a particular arrange ment of the longitudinal muscular fibres, which seem to form, when the creature advances, un dulations limited by the points of contact. These sections appear alternately to form a vacuum upon the surface where the animal is placed, that which follows advancing to take the place of that which precedes it, the trans mission of movement occurring from behind forwards, a mechanism which causes the animal to advance by a slow and uniform progression.