Structure of the tentacles of the Snail.

Fig. 265. Structure of the tentacles of the Snail.

(1403). We have already found that terrestrial species, such as the Snail, breathe air, which is alternately drawn into and expelled from a cavity lined with a vascular network; and these, from the resemblance between such a mode of breathing and that of animals possessed of proper lungs, have been formed into an order distinguished by the name of Pulmobranchiata. Nevertheless all the pulmobranchiate Gasteropoda are not terrestrial; our fresh waters abound with various species that respire air by a similar contrivance, and are consequently obliged, in order to breathe, to come continually to the surface of the shallow pools wherein they are found. The Planorbis and Limnaeus are examples of this mode of respiration, and are met with in every ditch, where they voraciously devour the sub-aquatic vegetables upon which they feed.

(1404). It is at once evident that in marine Gaste-ropods another mode of aerating the blood must be resorted to, and branchiae, of some description or other, substituted for a pulmonary cavity.

(1405). The branchiae given for this purpose are variously constructed - sometimes appearing as extensively branched and arborescent appendages to the skin, or else they form broad and thin lamellae attached to the exterior of the body; but more frequently the respiratory apparatus consists of vascular filaments arranged in a pectinated manner along a central stem. Whatever their form, however, their office is the same, namely to present a sufficient surface to the surrounding medium, in order adequately to expose the blood that circulates abundantly through them to the influence of oxygen.

Doris.

Fig. 266. Doris.

(1406). It is from the position and arrangement of the branchial organs that the branchiferous Gasteropoda have been classified by zoologists. Thus, in the second order, called from this circumstance NUDIBRANHIATA, they are naked, and placed upon some part of the back; sometimes, as in Tritonia, extending along its entire length; but in others, as for example in Doris (fig. 266), they are confined to its posterior part, and form a circle around the anal orifice, of exquisite beauty, and not inaptly comparable to a flower in appearance and disposition.

(1407). In the Inferobranchiata the branchiaa resemble two long rows of leaflets, placed on the two sides of the body, under a projecting edge formed by the mantle.

(1408). The Tectibranchiata have respiratory organs upon one side of the body only, and concealed by a flap derived from the mantle. Such, for instance, is the case with PleurobrancJius and Aplysia, in the former of which the elegant branchial fringe is situated in a deep sulcus between the edge of the mantle and the prominent margin of the foot (fig. 267, d).

Pleurobranchus.

Fig. 267. Pleurobranchus.

(1409). But by far the most numerous order of the marine Gasteropoda (Pectinibranchiata), which, in fact, includes all the inhabitants of spiral univalve sea-shells, have their branchiao placed internally in a capacious cavity, into which the water is freely admitted (fig. 262, a.) This cavity is situated in the last or widest turn of the shell, and communicates with the exterior of the body by a very wide slit, to which in some genera a long siphon (fig. 262,f), formed by a fold of the mantle or general covering of the animal, conducts the respired fluid. The branchiae themselves, as the name of the order indicates, are pectinated, and form a single, double, or triple series of gills suspended from the roof of the branchial chamber, answering the same intention as the pulmonary network of the Snail, but deriving their supply of air from the water in which they are perpetually immersed. In the figure referred to, representing a species of Pterocera, the position of the branchial chamber is seen through the shell and mantle, which the reader must suppose to be transparent; and the branchial organ (a), in this case single, is likewise represented in situ, suspended from the roof of the cavity that contains it.

(1410). In fig. 275 the roof of the respiratory cavity (a?) has been reflected, and the three rows of branchial fringes (n) suspended therefrom are well seen.

(1411). A sixth order of Gas-teropods has been formed by Cuvier under the name of Tubuli-branchiata, remarkable from the shape of their shells, which are long and irregular tubes, usually fixed to foreign bodies, but still they have the earliest-formed portion twisted into a few spiral curves. To this order belongs Vermetus (fig. 268), the shells of which, agglomerated into masses, might be taken for those of certain Serpulae. As locomotion is here out of the question, owing to the immoveable condition of the habitations of such genera, the foot would seem at first to be altogether deficient, but upon close inspection it is found to be converted into a fleshy organ that bends forward and projects beyond the head, where its extremity expands into a disk furnished with a small operculum; so that, when the animal retires into its abode, a lid is formed adapted to close the aperture, and thus prevent intrusion and annoyance from without. Nevertheless even in these the branchiae are pectiniform, forming a single row attached to the roof of a branchial chamber.

(1412). The Scutibranchiata likewise have pectinated gills disposed in a special cavity; but their shells are very wide, and scarcely ever turbinated - a circumstance which, combined with other features of their economy, renders it convenient to consider them as forming an order by themselves.

(1413). An eighth division of this extensive class takes the name of Cyclobranchiata, because the branchiae form a fringe around the body of the animal, between the edge of the body and the foot (fig. 260, c; fig. 263,a).