Vermetus.

Fig. 268. Vermetus.

(1414). Lastly, a distinct order has been established to embrace certain families in which the foot is so much compressed as to constitute a vertical muscular lamella, that presents merely a remnant of the ventral sucker so characteristic of the entire class, and which can only be serviceable in performing the office of a fin used in swimming; hence these mollusks have been called Heteropoda. Their branchiae are placed upon the back (fig. 269, d), and resemble small detached tufts. The form of these heteropod Gasteropoda the reader will gather from an inspection of the accompanying figure, representing a species of Pterotrachea; but the details connected with their anatomy, therein delineated, will be explained hereafter.

Pterotrachea.

Fig. 269. Pterotrachea.

(1415). It would be useless to weary the student by describing the course of the blood-vessels in all the orders we have just enumerated; their distribution necessarily varies with the changes observable in the position of the branchiae; still, whatever the situation of the respiratory organs, the general course of the circulation is the same, and essentially similar to what has been already described in the Snail: one or two examples will therefore answer our purpose. In the Pectinibranchiata, as for instance in Buccinum (fig. 275), the heart (r, s), enveloped in a distinct pericardium, is placed at the posterior extremity of the branchial chamber, and consists, as in all the Gasteropoda, of two cavities - a thin membranous auricle, and a more muscular and powerful ventricle. It receives the blood from the organs of respiration by a large branchial vein (fig. 275, q), that communicates with the auricle (s.) The contraction of the auricle forces the circulating fluid into the ventricle (r), which in turn drives it into the aortic or arterial system of vessels. The aorta, in the case before us, divides into two principal trunks, of which one (m) is directed forwards to supply the foot and anterior part of the body, while the other (t) winds among the mass of viscera contained in the shell, to which it distributes its ramifications. The blood thus dispersed through the system is taken up by the commencements of the veins, to be reconveyed to the branchiae, there to begin again the circuit we have described.

(1416). When the branchiae are external, and largely distributed over the surface of the body, as for instance in Tritonia, the purified blood is brought from the branchiae to the heart by capacious veins which run beneath each branchial fringe and collect it from the numerous respiratory tufts; or if, as in Boris (fig. 266), the branchiae encircle the anus, a large circular vein placed at the base of the branchial apparatus receives the blood and pours it into the auricle. In all cases, however, the course of the blood is essentially the same, and the heart is systemic.

(1417). In Aplysia, one of the tectibranchiate Gasteropods, the branchiae (fig. 270, a, b) consist of delicate lamellae minutely subdivided; and the vessel (c) which brings the blood derived from all parts of the body to be distributed over the extensive surface thus formed, presents a structure of no ordinary interest to the physiologist*. At some distance before it arrives at the respiratory organs it divides into two main branches; and the coats of each vessel so formed appear to be made up of transverse and oblique muscular bands that cross each other in all directions, so as to leave between them very perceptible apertures, through which injections of any kind readily escape into the abdominal cavity, and, of course, fluids derived from the abdomen as easily penetrate into the interior of the veins. At some points, indeed, these veins seem absolutely confounded with the visceral cavity, - a few muscular bands widely separated from each other, and not at all interrupting a free communication, being alone interposed. The result of Cuvier's anxious researches concerning this remarkable feature in the organization of these Mollusca led him to the following important conclusions, which are no doubt extensively applicable to the Gasteropoda generally: - 1. That in Aplysia there are no other vessels appointed to convey the blood to the branchiae than the two above described. 2. That all the veins of the body terminate in these two canals.

Now, as their communication with the abdominal cavity is evident and palpable, whether we call them venae. cavae, or cavities analogous to a right ventricle, or branchial arteries, - for it is manifest that they fulfil the functions of these three organs, - the inevitable conclusion is, that fluids poured into the abdominal cavity can become directly mixed with the mass of the blood and thus conveyed to the branchiae, and that the veins perform the office of absorbent vessels.

* Cuvier, Mernoire sur le Genre Aplysia.

(1418). This extensive communication is undoubtedly a first step towards the establishment of that, still more complete, which nature has established in Insects, where, as we have seen, there are not even distinct vessels of any kind appointed for taking up the nutritive fluid. From these facts Cuvier concludes that no proper absorbent system exists in the Mollusca, still less in animals inferior to them in the scale of creation.

(1419). The vein appointed to convey the renovated blood from the branchia) to the heart, when slit open (fig. 270, d), exhibits the orifices of the smaller vessels derived from the respiratory laminae arranged in circles. The auricle of the heart is made up of reticulated fibres (e); and when laid open it is seen to be separated from the more muscular ventricle (g) by a valve (f), whereby any retrograde movement of the blood is prevented.

Vena cava of Aplysia laid open.