The heart in this class of animals is ex tremely small in proportion to the body. Its structure is very simple, as it consists of a single auricle and ventricle, which correspond with the right side of the heart in warm-blooded animals. The ventricle gives rise to a single arterial trunk,(which is expanded in most fishes into a kind of bulb as it leaves the heart,) going straight forwards to the branchim, or organs of re spiration. The blood passes from these into a large artery, analogous to the aorta, which goes along the spine, and supplies the body of the animal. It is then returned by the venx caves into the auricle.
It appears that insects possess neither blood-vessels nor absorbents. Cuvier has examined, by means of the microscope, all those organs in this class, which in red blooded animals are most vascular, with out discovering the least appearance of a blood-vessel, although extremely minute ramifications of the trachea are obvious in every part. And Lyonet has traced and delineated in the caterpillar, parts infi nitely smaller than the chief blood-vessels must be, if any such existed. " Anatomie de la Chenille," &c.
Yet insects, bOth in their perfect and in their larva state, have a membranous tube running along the back, in which al ternate dilatations and contractions may be discerned. From this circumstance it has been supposed to be the heart ; but it is closed at both ends, and no vessels can be perceived to originate from it.
It is obvious from these data, that the functions of nutrition and secretion must be performed, in the animals which we are now considering, in a very different man ner from that which obtains in the more perfect classes. Cuvier expresses the mode in which he supposes growth and nutrition to be effected, by the term " im bibition." And he explains, from this cir cumstance, the peculiar kind of respirati on which insects enjoy. Since the nutritive fluids have not been exposed to the atmo sphere, before they arrive at the parts for whose nourishment they are destined, this exposure is effected in the parts them selves by means of the air-vessels, which ramify most minutely over the whole body.
" En un mot, le sang ne pouvant alter chercher l'air, c'est Pair, qui va chercher le sang." The heart of the crustacea, according to Cuvier, has no auricle ; and it is what he calls an aortic heart. For it expels the blood into the arteries of the body ; and this fluid passes through the gills previ ously to reaching the heart again. The different parts of the system are here found under a mode of connection exactly the reverse of what we observe in fishes, where the blood is sent into the gills, and passes subsequently into the aorta. The circulating organ in that class is therefore a pulmonary heart.
According to Cuvier, the cuttlefish has three hearts, neither of which pos sesses an auricle. Two of these organs are placed at the root of the two bran chim : they receive the blood from the body, (the vena cava dividing into two branches, one for each lateral heart,) and propel it into the branchia. The returning veins open into the middle heart ; from which the aorta proceeds.
The other mollusca have a simple heart, consisting of one auricle and ven tricle. The vena cava assumes the office of an artery, and carries the returning blood to the gills ; whence it passes to the auricle ; and is subsequently ex pelled into the aorta. Here therefore, as in the crustacea, the heart is a pulmo nary one.
The vermes of Cuvier have circulat ing vessels, in which contraction and di latation are perceptible, without any heart. They can be seen very plainly in the lumbricus marinus. The leech, naias, nereis, aphrodite, &c. are further exam ples of the same structure. This anato mist is of opinion that the mollusca, crus tacea, and vermes, possess no absorbing vessels ; and he thinks that the veins ab sorb, as he finds them to have communi cation with the general cavity of the bo dy, particularly in the cuttle-fish. Hence the above mentioned classes will hold an intermediate rank between the vertebral animals, which possess both blood-vessels and absorbents, and the insects, which have neither.