Having thus briefly described the origin of the different parts of a shell in the simple form which we have chosen as an example, we shall now proceed to examine the structure and mode of growth in others of a more complicated aspect. The majority of the Gasteropoda are furnished with a shell which has been denominated spiri valve. Let the reader imagine the shell of the Patella to be lengthened into a long cone, which, instead of preserving its symmetrical form, is twisted around a central axis, and lie will imme diately understand the general arrangement of the parts in shells of this description. The cause of such an arrangement is owing to the shape of the body of the animal inhabiting the shell, which, as it grows, principally enlarges its shell in one direction, thus of course making it form a spire modified in shape according to the de gree in which each successive turn surpasses in bulk that which preceded it. The axis around which the spire revolves is called the columella, and the mode of revolution around this centre gives rise to endless diversity in the external form. In the spirivalve-shelled Gastcropoda, as in those last described, we find a difference in structure between that part of the mantle which envelopes the viscera, and is always concealed within the cavity of the shell, and the more vascular portion placed around its aperture : the former is thin and membranous, its office being merely that of thickening the shell by the deposition of successive calcareous strata applied to its inner side, and of producing the pearly lining which smooths and polishes the interior ; the latter part of the mantle is thick, spongy, and coloured, secreting largely the cal careous particles with which the progressive amplification of the shell is effected : this por tion (fig. 179, c,) from its thickness, and the manner in which it usually surrounds the en trance to the shell, is generally termed the col lar. In such species as inhabit coloured shells we may observe upon the surface of the collar (fig.179, d,) patches of different colours corres ponding in tint with the various hues seen upon the exterior. These spots supply the pigment, which being mixed up with the earthy cement serving for the enlargement of the shell stains it with a corresponding tint. In many instances, as in the figure, the colours are continually secreted by the dark spaces, d, causing the painted bands which they produce to wind un interruptedly in the direction of the convolu tions of the spire, and they may be seen gra dually to increase in breadth as the size of the animal enlarges : but more frequently it happens that the colouring matter is only furnished at stated periods, and in such cases of course the shell will be marked with spots, the intervals be tween which will be regulated by the frequency of the supply. It will be seen that by a combina tion of these circumstances it is easy to explain how every variety of marking may be produced.
The most conspicuous exception to the gene ral process by which shells are painted, is met with in the porcellaneous Cowles ( Cypraw), which at various periods of their growth could scarcely be recognised as belonging to the same genus. In the young animal the enlargement of the shell is effected in the ordinary manner, and its colours are supplied from the surface of the collar : in the mature state, however, these shells are coloured in a very different manner, and acquire at the same time a great increase of thickness; this is effected by the enormous de velopment of the aloe of the mantle, which in the full-grown animal become so much ex tended, that when the creature is in motion they are laid over the external surface of the shell so as entirely to conceal it. These alm contain patches of pigment which secrete colours en tirely different from those contained in the collar, and from their whole surface exudes a calcareous varnish, which being laid over the exterior of the old shell completely conceals the original markings ; these, however, may he again exposed on removing with a file the outer crust : a line, which is generally very distinctly seen running longitudinally along the back of the shell, indicates the spot where the edges of the two aim of the mantle met during the com pletion of this singular process. Such shells
are therefore remarkable from the circumstance of having their thickness increased by additions to the outer as well as to the internal surface.
In terrestrial shells it is only when they have arrived at their full growth that a rim or margin is formed around the aperture, which serves to strengthen the whole fabric; but in marine shells, which attain to much larger dimensions, the growth is effected at distinct periods, each of which is indicated by a well-defined margin, and these ridges remaining permanent, the suc cessive stages of increase may be readily seen. At each suspension of development, it is not unusual to find spines or fringes, sometimes differently coloured from the rest of the shell, and not unfrequently of considerable length. Infig. 180, which represents the shell of Murex enrnutus, the nature and arrangement of such spines is well exemplified. They are all formed by the margin of the mantle which shoots out into long fringes, encrusting themselves with a shelly covering ; each spine therefore is at first hollow, and if in many species they are found solid, it is because the original cavity has been gradually filled up by the deposition of earthy matter within it. The syphon with which many Conchiferous Gasteropoda are provided is pro duced in precisely the same manner, and its identity in form with the other spines covering the surface of the shell is in the annexed figure sufficiently obvious. In many species, as in the beautiful Turbo scolaris, (fig. 181,) the epochs of growth are only indicated by ridges surrounding the shell at regular intervals, each of which originally terminated a fresh augmen tation of its size. It is difficult to imagine by what influence these creatures are induced to enlarge their habitations at such regular inter vals, terminating each operation by a similar margin ; some authors imagine that each time the creature emerges from its abode a fresh addition is made ; others that it is dependent upon the temperature or state of the seasons, but without sufficient grounds for either of these assertions ; it seems more probable therefore that the growth of the body gradually rendering the former dimensions of the shell incotnmo dious from time to time renders these pe riodical enlargements necessary.
Although shells are evidently inorga nic and extra-vascu lar structures, it is now universally con ceded that their in habitants have the power of removing portions which may obstruct their growth, or needlessly infringe upon the limits of their abode. the Murices we have in-.
disputable evidence of this fact in the removal of such spines as would interfere with the revolutions of the shell around the columella, and in Conus and similar genera a like faculty enables the animals to thin the walls which bound the inner whirls when their original thickness is rendered un necessary by the accession of new turns. Such a solvent power indeed is not only exer cised upon their own habitations, but many Gasteropods are able gradually to bore holes in other shells, or perforate the rocks upon which they reside to a considerable depth. The mode in which this is effected is, however, still a mystery ; some authors ascribe it to a power of absorbing their shells, an expression the vagueness of which is sufficiently evident ; others ascribe it. to some acid secretion at the disposal of the animal ; yet although this ex planation is certainly plausible, when we reflect that the very structure which secretes this sup posed acid is itself the matrix of such abundant alkaline products, it is not easy to imagine how the same structure can at the same time furnish such opposite materials.