Respiration is effected in the Pulmonary Gasteropoda, whether they be terrestrial or aquatic, by an apparatus fitted for breathing the air of the atmosphere ; the lung or puhno-bran chia, as we may call this singular organ, con sists of a large cavity placed beneath the man tle, over the surface of which the vessels return ing the blood from the system spread in beau tiful ramifications, and from these the pulmo nary veins take their origin, collecting the blood which has been exposed to the action of the air, and conveying it to the heart. A large orifice admits the air freely into this chamber, the walls of which alternately contracting, draw in and expel it at regular intervals by an action precisely similar to that of the human dia phragm. In the Slugs (Limax) the cavity is small, but the network of the vessels spreads over its whole surface. In the Snail (Helix), on the contrary, the organ is much larger, but its floor only is covered with the respiratory ramifications. In fig. 322, of the article CIR CULATION, vol. i. p. 649, a diagram is given of this structure, and in fig. 190, (m, n,) the details of its arrangement are more minutely shewn; yet even in the beautiful drawing of Cuvier, from which our plate is copied, the minute divisions of this superb plexus are but inadequately shewn. The order which has been established by Ferussac, under the name of Pulmonalia operculata, is composed of individuals classed by Cuvier among the Pcctinibranchinfa, to which in every cir cumstance, with the exception of the struc ture of the respiratory system, they are closely allied ; these, however, breathe the air in a cavity analogous to that which we have just described, only differing in the position and nature of the aperture leading to it, which here, instead of being a rounded orifice in the margin of the collar, opened and closed at the will of the animal, is a large fissure placed above the head, exactly as in the Pectinibranchiate order.
Organs of circulation.—Having thus de scribed the different arrangements of the branchias, we shall be enabled more readily to investigate those modifications in the dis position of the organs subservient to the cir culation of the blood which are dependent thereupon. Throughout the whole class, with the exception of the Seutibranchiate and some of the Cyclobranchiate orders, the heart is single, consisting of an auricular and ventri cular cavity, and is interposed between the branehial or pulmonary vessels and the system, receiving the aerated blood from the respiratory organs, and propelling it through the body. The heart of Aplysia (fig. 191, e, g ) or of the Snail, (fig. 190, o, p) will exemplify its ordi nary structure. The auricle varies slightly in shape in different genera, but is always ex tremely thin and pellucid, containing in its coats muscular bands of great delicacy. The ventricle is provided with stronger walls, and is generally separated from the auricle by a valve, formed of two pieces. The heart is en closed in a pericardium, but its position is re gulated by that of the branchim; and from the great diversity of arrangement which we have found the latter to present, a corresponding want of uniformity in the locality which the heart occupies may be readily expected. We shall select two forms of the respiratory organ as examples of the variable position of the heart, and as illustrations of the usual distribu tion of the bloodvessels, viz. the Snail, (vide CIRCULATION, fig. 322, and the Doris, Jig. 321,) and afterwards notice the principal aber rations from the ordinary disposition. In the Snail, the blood derived from the whole body is brought by great veins, performing the func lions both of the vena cava and of a pulmonary artery, to the plexus of vessels lining the floor of the respiratory cavity ; after here undergoing the needful aeration, it enters the heart, from whence it is driven into the aorta. The aorta immediately divides into two trunks, one dis tributed to the liver, the intestine, and the ovary; the other supplying the stomach, the oral apparatus, the organs of generation, and the foot. In the Slug the arteries are perfectly white and opaque, and their ramifications, which may be traced with great readiness, are extremely beautiful.
In Doris (fig. 321) the heart is, in conse quence of the position of the branchize around the anus, removed quite to the posterior extre mity of the body. The blood derived from all parts of the body is conducted by large veins to the respiratory organs; the pulmonary arte ries which return it from thence unite into a circular vessel (b, b), surrounding the anus, and from this arise two vessels, emptying them selves into the auricle. The aorta, on issuing
from the heart, divides into two large vessels, the first supplying the intestinal canal, stomach, and duodenum, the organs of generation, the foot, and the mouth ; whilst the other large trunk is entirely distributed to the liver.
In Trilonia the heart is placed near the centre of the body, and the auricle itself resem bles a cylindrical vessel placed transversely across the other viscera, and communicating with the ventricle near its middle. The blood arrives at the heart through four vessels from the long fringe of branchiw, two coming from the anterior and two from the posterior parts. We have already described the disposition of the branchim in the Tectibranchiate order, but in following the course of the circulating fluid, we shall find in some of the individuals in cluded in this division circumstances requiring special notice, as being of extreme interest to the physiologist. In Aplysia, the blood re turned from the system is brought by two large venous trunks to the vena cava or pulmonary artery (fig. 1'91, Ii); for in this case the same vessel performs the functions of both; these large veins turn round in the vicinity of the operculum, and unite into one trunk prior to their dispersion over the branchial plates, but on opening them at this point so as to display their interior, a most singular arrangement is brought to light; the sides of the veins are found to be formed of muscular bands (c) crossing each other in various directions, and leaving spaces between them ; these intervals are seen even by the naked eye to he apertures establishing a free communication between the interior of the vein and the abdominal cavity, and allowing injection to pass with facility from the vein into the visceral cavity, or from the abdomen into the vein : the anterior portion of each of these vessels may indeed be said to be literally confounded with the general cavity of the the body, a few muscular bands, forming no obstacle to a perfect communication, being the only separation between the two. It is there fore evident that the fluids contained in the abdominal cavity may in this manner have free access to the mass of the blood as it approaches the respiratory organ, and that the veins can thus perform the office of the ab sorbent system ; but in what manner the blood is prevented from escaping through the same channels is not at all obvious, although pro bably during life the contraction of the fasci culi which bound these apertures may in some measure obstruct the intercourse. It is from this circumstance, and the analogous commu nication which exists in the Cephalopoda by the intervention of the spongy appendages to the vena: caves found in those Mollusks, that Cuvier was led to the conclusion that in all the class the veins are the immediate agents of absorption, and that an absorbent system does not exist in any but the vertebrate division of the animal kingdom. We meet, moreover, in Aplysia with another peculiarity in the cir culating vessels; the aorta, shortly after its commencement, divides into two large arteries one of which presents nothing peculiar in its distribution; but to the larger of the two, whilst still enclosed in the pericardium, we find appended a remarkable structure, the use of which has been hitherto perfectly inexpli cable : projecting from the opposite sides of the vessel are two vascular crests, represented in i, formed of aplexus of vessels issuing from the aorta itself, and ramifying in an ex ceedingly beautiful manner through the sub stance of these extraordinary organs; in other respects the arteries are distributed in the usual manner. The Cyclobranchiate and Scutibran chiate Gasteropods approximate the testaceous class in many points of their organization, but in none more so than in the position which the heart is found to occupy, and the arrangement of its cavities. In Patella, indeed, the heart is placed in the anterior part of the body, and still conforms in its general structure to the description which we have given above ; but in Oscabrio the auricle is divided into two distinct portions, one receiving the blood from each range of branchial plates ; and in Haliotis, Fissurella, Emargenula, and Parmophorus, not only is this division of the auricle met with, but the ventricle, as in many of the testaceous Mollusks, is perforated by the rectum, and the similarity of arrangement which is here presented with what is met with in the Con chifera will be readily appreciated by a refer ence to the article which treats of the anatomy of that division of the Mollusca.