382 



GASTEROPODA. 



ciferous membrane. The colouring matter 

 becomes thus incorporated at definite points, 

 with the cement by which the shell is extended, 

 and is arranged in various manners according 

 to the position of the secreting organs which 

 furnish it. Another peculiarity which distin- 

 guishes external shells is that their outer sur- 

 face is often invested with a membranous layer, 

 called the epidermis, which having been re- 

 garded by some authors as a part of the true 

 integument of the body, has given rise to the 

 supposition that all shells being placed between 

 two layers of the skin were in fact internal, the 

 difference between the one and the other con- 

 sisting merely in the extent of development. 

 In support of this opinion reference has been 

 made to the great thickness of this epidermic 

 coat, which not unfrequently is such as to give 

 to the surface of the shell a felted or pilous ap- 

 pearance ; but if such an idea were correct, it 

 is evident that the epidermis must be formed 

 prior to the deposit of calcareous matter be- 

 neath it, which observation has disproved, in- 

 asmuch as those shells in which the epidermic 

 covering is most dense and shaggy are found 

 whilst in ovo to be without such an investment. 

 The so-called epidermis, therefore, whatever 

 may be the aspect which it presents, whether it 

 be, as is usually the case, a brittle lamella en- 

 crusting the shell, or a flocculent and pilous 

 covering, is evidently inorganic, being merely a 

 crust of inspissated mucus, originally secreted 

 with the calcareous particles, and forming when 

 dry a layer encrusting the surface of the shell. 



There is yet another structure common to 

 shells of this class, of which it remains to speak, 

 namely, the enamel or pearl, which lines such por- 

 tions of them as are immediately in contact with 

 the body of the animal ; this polished material 

 may be likened to the glazing of an earthen- 

 ware vessel, and is a varnish produced from 

 the general surface of the mantle, by some mo- 

 dification of its secretion the nature of which 

 is unknown, and spread in successive coatings 

 over the more coarse calcareous matter, where- 

 ever such a polish becomes needful. 



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 



frowth in others of a more complicated aspect, 

 'he 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 he 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 coktmeUa, 

 and the mode of revolution around this centre 

 gives rise to endless diversity in the external 



form. In the spirivalve-shelled Gasteropoda, 

 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 (Jig. 179, c,J from its thickness, and the 



Fig. 179. 



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 

 (Jig. 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 Couries ( Cyprcea), 

 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 alae 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 alae contain 

 patches of pigment which secrete colours en- 

 tirely different from those contained in the 

 collar, arid from their whole surface exudes a 



