From the modifications observable in the structure of the integument, it is not to be won dered at that names have been applied to diffe rent portions, which it will be useful to notice,. especially as they are not unfrequently used in a confused and unprecise manner. That por tion of the skin which is more immediately connected with the secretion of the shell, in such Gasteropoda as are provided with a de fence of that description, has been termed the mantle, and in certain instances, from the mode in which it seems to form a special cos cring to a part of the body, it has some claim to the name; the mantle is, however, extremely varia ble, both in position and arrangement. In the Nudibranchiata, which have no shell, it cannot be said to exist, as no fold of the integument or defined margin indicating a portion deserving of a distinct appellation can be detected. In the Teclibranchiata the mantle is a small trian gular fold of the integument on the right side of the body, inclosing a rudimentary shell, and serving as a covering to the subjacent branchiw. In the Inferobranchiata it invests the whole of the back, and forms a fold around the margins of the body, beneath which the branchiw are found ; whilst in all the conchiferous Gastero pods it lines the interior of the shell, whatever its shape, forming a distinct fold or thickened rim around its aperture, to which when much developed, as in helix, the name of collar is not improperly applied.
In the naked terrestrial species the mantle consists of a thickened portion, occupying a variable position on the back, and more or less defined by a distinct margin ; it is here not un frequently termed the corselet, and generally contains a calcareous plate. In Vaginula it covers the whole of the back; in Liman it occu pies only its anterior portion; in Parmacella it is found in the middle of the dorsal region, whilst in Testacella it is placed quite poste riorly in the vicinity of the tail; yet whatever its situation, shape, or size, it is the immediate agent in the formation of the shell, and as such we have deemed it necessary to be thus precise in describing the different aspects which it assumes.
Growth of shell.—The varied and beautiful shells that form so important a part of the inte gument of many individuals belonging to this order, however they may differ in external form and apparent complication, are essentially simi lar in composition and in the manner of their growth. These calcareous defences, although serving in many cases as a support to the ani mal, from which important muscles take their origin, differ widely from the internal skeletons of vertebrate animals, being mere excretions from the surface of the body, absolutely extra vital and extra-vascular, their growth being en tirely carried on by the addition of calcareous particles deposited in consecutive layers. The dermis or vascular portion of the integument is the secreting organ which furnishes the earthy matter, pouring it out apparently from any part of the surface of the body, although the thicker portion, distinguished by the appellation of the mantle, is more especially adapted to its pro duction. The calcareous matter is never depo sited in the areolm of the dermis itself, but ex udes from the surface, suspended in the mucus which is so copiously poured out from the mu ciparous pores, and gradually hardening by ex posure; this calciferous fluid forms a layer of shell, coating the inner surface of the pre-exist ent layers to increase the size of the original shell, or else is furnished at particular points for the reparation of injuries which accident may have occasioned. It is to the insestiga
lions of Reaurnur that we are indebted for our knowledge concerning this interesting process, and subsequent writers have added little to the information derived from his researches; in order, however, to lay before the reader the principal facts connected with this subject, we shall commence with the simplest forms of the process, and gradually advance towards such as are more complicated and less easily under-. stood.
The shells of the Gasteropoda are of two kinds, some being entirely concealed within the substance of the mantle, and consequently internal, whilst others are placed upon the sur face of the body external to the soft Integument. In the former case the shell is uniform in tex ture and colourless ; in the latter, its develope ment is much more elaborate, and it is not un frequently moulded into a great diversity of forms, and painted with various tints, which are sometimes of great brilliancy. The internal or dermic shells are found in many of the pulmo nary and teclibranchiate orders, and possess but little solidity ; although inclosed in the substance of the mantle, they are so little adhe rent, that when exposed by an incision they readily fall out of the cavity in which they are lodged, and from which they are apparently quite detached. Their substance is generally calcareous, but in many instances, as in Aplysia, the shell is of a horny texture, being transpa rent, flexible, and elastic, as is the gladius of many of the Cephalopod Mollusca. In all cases horny or calcareous plates of this descrip tion are found to be composed of superposed lamella?, which are successively secreted by the floor of the cavity in which they are contained, the inferior layer being always the largest and most recent. These shells, therefore, may be con sidered as merely formed by the deposition of successive coats of varnish, which become indu rated, and the simple manner of their growth will best exemplify the mode in which more compli cated shells, whatever be their form, are con structed. External shells present an endless di versity of figure, and some classification of their principal forms will facilitate our contemplation of the peculiarity observable in each. The con cealed shells, which are merely the rudiments of wltat we are now considering,are so small in com parison with the site of the body, that they can only be looked upon as serving for the protec tion of the more important organs, namely, the heartand respiratory apparatus,which are placed beneath them, but the external shells, from their great developement, are not merely a partial protection to the animal, but in most cases constitute an abode into which the creature can retract its whole body. The external shell consists generally of one piece, the form of which may be symmetrical, in which case it is a cone or disc simply covering the back of the animal ; or, as is generally the case, the shell may be more or less twisted around a central axis, forming a convoluted, turbinated, or spiri valve shell. In one genus only, Chilon, Lin., the shell is formed of several pieces articulated with each other, and covering the surface of the back.