(1427). Lastly, it may be noticed that in the cephalic region, where the different organs are in immediate contact with the arterial blood, no traces are discernible either of veins or of lacunae serving to return the blood thus effused to the respiratory apparatus, whereas in other parts of the body venous canals are met with, the disposition of which is very remarkable: these all communicate freely with the abdominal cavity, as is the case in other Gasteropod Mollusca; but in the liver, the generative glands, and more especially in the urinary apparatus, they nevertheless form true vessels, the ramifications of which are extremely numerous.

(1428). In Patella, or the Limpet, the size of the cephalic sinus that receives blood from the aorta is even more remarkable than in the Hallotis: in the Patella, indeed, the tongue is not itself lodged in the aorta, as in the former case, but is enclosed in a membranous sheath; the sheath, however, in its turn becomes part of an arterial chamber, into which the aorta empties itself. The aorta itself gives off very few branches, while from the lingual sheath arise all the principal arteries of the body.

(1429). The arterial blood fills not only the sheath of the tongue, but is likewise diffused throughout the whole cephalic cavity, where it bathes the muscles and nerves in the same manner as in Haliotis; but the extent of the sanguiferous sinus is much more considerable than in that mollusk. If, indeed, the capacity of these sinuses be estimated, they will be found to contain more blood than all the rest of the arterial system put together.

(1430). Such is the construction of the heart in a great majority of the Gasteropoda; but in a few of the lowest orders, namely those most nearly allied to the Conchifera, slight modifications are met with. Thus, in Chiton (fig. 263), so remarkable from the singularity of its shelly covering, the heart is situated in the middle of the posterior region of the back, and is furnished with two auricles, one appropriated to each lateral series of branchiae; and, what is still more remarkable, each auricle would seem to communicate with the ventricle by two distinct orifices. In Haliotis, Fissurella, and others of the Scutibran-chiate and Cyclobranchiate orders, the resemblance to the arrangement generally met with among the Conchifeea is even more striking; for in such genera not only are there two distinct auricles, but the ventricle embraces the rectum, so that, when superficially examined, it seems to be perforated for the passage of the intestine.

(1431). In Pterotrachea (fig. 269), the branchiae (d) are placed upon the back, and the blood derived from the tufts composing the branchial apparatus is received into a two-chambered heart (e), whence it is distributed to the body through the aorta, which is at first double; but, after surrounding the visceral sac and supplying the viscera, the two vessels unite to form one large trunk (m), which traverses the body as far as the head.

(1432). Independent of the ordinary vascular system, Delle Chiaje discovered the existence in most Gasteropods of a system of water-vessels largely distributed throughout the substance of the foot and other parts of the body. Thus, in the anterior part of the foot of the Muricidaa*, there are to be seen certain holes or antra, which are the apertures to as many little cavities lying underneath, and which permeate the interior substance of the foot. There are, besides, between these cavities slender canals communicating with the same orifices, by means of which the whole are connected and inosculated together. The water entering the body through the siphuncle is thus, at the will of the animal, driven into the substance of the foot, which is in this way rendered turgid and firm; and when necessary, by a strong pressure, the fluid is ejected, or is spontaneously discharged after death, when the foot becomes flaccid and extenuate. Opinions relative to the use of the water thus freely admitted into the body of the Hollusca are various; its principal object, however, seems to be to enlarge and moisten the structures over which it is distributed.

(1433). The digestive system of the Gasteropoda, as we might be led to expect from the numerous and widely-different forms of the animals belonging to the class under consideration, presents endless diversity of structure; and did we not strictly refrain from noticing any but the most important modifications, it would be easy to overwhelm the most patient reader with accumulated details.

* Delle Chiaje, Anim. senza Vert. d. Nap. ii. p. 204.

(1434). The mouth we shall consider as exhibiting four distinct types of organization; one of which, namely that met with in the Snail and the generality of pulmonated Gasteropoda, has been already described (§ 1380).

(1435). The second form of mouth - that, for instance, of Pleuro-brancJius (fig. 267, a) and of Pterotrachea (fig. 269, b) - consists of a simple muscular proboscis, or fleshy tube, which is capable of considerable elongation and contraction: such an oral apparatus is entirely devoid of teeth or any cutting instrument, but is nevertheless fully able to seize and force into the stomach such materials as are used for food.

(1436). A third kind of mouth, by no means so frequently met with as the last, is not a little extraordinary, and forms a more efficient cutting instrument than even that of the Snail. We shall offer, as an example of this remarkable organ, that of the Tritonia Hombergii, represented in the annexed figure (fig. 272), whereof Cuvier gives the following graphic description*. In this animal the mouth forms a large oval and fleshy mass enclosing the jaws and their muscles, as well as a tongue covered with spines; and its opening is guarded by two fleshy lips. The jaws form the basis of all this apparatus: their substance is horny; their colour a yellowish brown; and their form (very extraordinary for an organ of this kind) cannot be better described than by comparing them to the shears used in shearing sheep. They differ, however, in the following particulars: - instead of playing upon a common spring, the two blades are found to work upon a joint, and, instead of being flat, they are slightly curved.