Order VIII. CYCLOBRANCIIIATA, (Cuv.) Syn. Dermobranches, Dum.; Gust. Phylli diens, Lam.; Gast. Chisrnobranches, Blain.
In this order the branchiw are arranged under the margin of the mantle around the circumference of the body ; the shell is a simple shield, either composed of one piece, which is never turbinated, or else made up of several divisions. They are all hermaphrodite and self-impregnating.
1st Sub-order, Chismobranchiata, Blain.; Cy clobranchiata, Goldfuss.
1st Faro. Patella.
2d Sub-order, Polyplariphora,! Blain.
2d Fam. Oscabrion.
Cuvier detaches the genera Vermetus, .illagi lus, and Siliquaria from the I'ectinibranchiata on account of the irregular form of their shell, which is only spiral at its commencement, and is usually firmly attached to some foreign body, a circumstance which involves as a necessary consequence the hermaphrodite type of the sexual organs, so that these genera are self impregnating. lie has, therefore, arranged them in a separate order, to which lie applies the name of Tubulibranehiata.
Tegumentary system.--The skin which in vests the Gasteropoda varies exceedingly in texture, not only in different species but in dif ferent parts of the same animal ; its structure being modified by a variety of circumstances connected with the habits of the creature, the presence or absence of a calcareous covering, or the mode of respiration. In the naked Gaste ropods, especially in the terrestrial species, it is thick and rugose, serving as a protection against the vicissitudes consequent upon the changeable medium which they inhabit. In such as are aquatic the integument is proportionably thin ner, and its surface more smooth and even ; in both, however, it differs much in texture in dif ferent parts of the body ; thus in the dertno branchiate species it becomes attenuated into a thin film, where it invests the vascular appen dages subservient to respiration, and such por tions as cover the organs of sense assume a transparency and delicacy adapted to the sen sibility of the parts beneath. In those orders which are provided with shells, the integument which protects such parts of the body as are exposed when the animal partially emerges from its abode, is thick and spongy, and very different from the thin fibrous membrane which invests the mass of viscera contained within the shell. We are led by various circum stances to presume that the skin of all the Gasteropods is in structure essentially ana logous to that of higher animals, and in de-.
scribing it we shall avoid obscurity by applying to its different parts the names ordinarily made use of by anatomists to distinguish the tissues enumerated as composing the human integu ment.
The dermis is an extremely lax and cellular texture, eminently elastic, and so intimately blended with the contractile layers beneath it, that it is difficult to recognise it as a distinct structure : its great peculiarity consists in the power which it possesses of secreting calcareous matter, which being deposited either in a cavity within its substance, or as is more frequently the case, upon its outer surface, forms a con cealed or external shell : from this circum stance, and from the abundant quantity of mucus which it constantly furnishes, we may infer its great vascularity, while the high degree of sensibility which it evidently possesses une quivocally demonstrates that it is plentifully supplied with nerves, although the existence of a true papillary structure cannot be satisfac torily distinguished. The colouring pigment likewise exists, as is evident from the brilliant markings which are often met with in some of the more highly coloured species ; but there is a circumstance in connection with this rete mucosum which requires particular mention, as it will enable us afterwards more clearly to ex plain the formation of shells; the pigment is not merely a layer which serves to paint the surface of the body generally, but appears ra ther to be an infiltration of the lax tissue of the cutis with coloured fluid, which is poured out in great abundance at particular points, espe cially around the margin of the shell, and there being mixed up with the calcareous matter se creted by the collar, its tints are transferred to the exterior of the shell itself, tinging it with similar hues. The epidermis is evidently defi cient, its place being supplied by the viscid matter with which the surface of the body is continually lubricated. The muciparous crypts destihed to furnish the copious supply of glairy fluid with which the skin is so largely moist ened, have not been detected, but the pores through which it exudes are sufficiently distinct. It is in connexion with the needful diffusion of this secretion over the entire animal, that the skin of the terrestrial species, as the Slugs and Snails, is observed to be deeply fiirrowed by large anastomosing channels, formed by the rugw of the surface, and serving as canals for its conveyance by a species of irrigation to every point. No pilous system, properly so called, exists in any of the Gasteropods, the hairy covering of many shells being, as we shall presently see, of a widely different nature.