Tlie blood, after having been arterialized its passage through tlte capilktries of the branchix, is wured into the efferent vessel, which, as we shall immediately have occasion to see when tileating of the respiratory process, runs along the internal surface of each bran chia. It enters the thoracic cells in the same manner as the afferent vessel passed out from them, bends upwardly under the vault of the flancs, and thus takes its course towards the heart. It is to this portion of the canal that we have given the name of brandio-cardiac v.essel.
The mode in which the blood enters the heart is still a subject under discussion. Our in quiries lead us to believe that this fluid, poured by the branchio-cardiac canals into a sinus situated on each side of the heart, penetrates this organ by means of certain openings situated in those parts of its substance which are directly opposite to the canals mentioned. But Messrs. Lund and Strauss imagine that the blood is effused as it were into the peri cardium (which is named auricle by the latter anatomist) to penetrate from thence by open ings situated on the superior surface of the heart.* These openings, however, we conceive to be closed in the natural state by means of a membrane, and it is also worthy of remark that the writers just cited were unacquainted with the lateral openings which establish a much more direct communication with the interior of the organ. We must also add that the celebrated John Hunter, whose labours upon this subject have hitherto remained un known to the world, but which have very re cently been given to the public by Mr. Owen, had long. ago ascertained the existence of the venous sinuses and of the lateral openings of the heart, although he seems to have thought that the circulation was not complete in the manner we have described it.* In the most inferior groups of the class of Crustaceans the apparatus of the circulation becomes much less perfect, and even seems to disappear entirely in the last of the Haustel late tribes. In the Argula, for instance, there still exists a heart, but the arteries as well as the veins appear to be nothing more than simple lacunm, formed in the interstices between the different org,ans; and in the Nicothoa, &c. no distinct trace of any portion of a circulatory system has yet been discovered.
C. Of the respiration.—The Crustacea, like all the other tribes especially formed for living under water, respire by means of certain parts of their external covering modified in its struc ture in order to fit it for this function, and known under the name of branchi&. This chara.cter is even so completely inherent in the organic type proper to this class, that it is still preserved in certain species which live on the land and not in the water.
Nothing, however, can be conceived more various than the form and disposition of the organs of the branchial respiration among these animals : in some the function is per formed by an extremely complex apparatus, consisting in great part of organs created ex pressly for this end ; in others it is delegated to certain appendages which do not exist for the office exclusively, but are rather turned from their more ordinary and obvious uses to subserve this important function. In others still, we neither discover special organs of respiration nor other parts whose structure fits them evidently to supply the place of branchiw; in these cases we can only suppose that the oxygen held in solution by the water acts upon the nutritious fluid of the animal by the inter medium of the entire tegumentary covexing.
Let us first review the respiratory apparatus in its state of greatest complexity, but com mencing. with it in the embryo and following it in its progressive development, in order that we may he the better prepared to compare it with those forms which vvill be presented to us among species less elevated in the series of the Crustaceans.
In the earliest periods of embryotic life of the COMMOD Astacus fluviatilis, we discover no trace of branchix; but at a somewhat more advanced stage of the incubation, though still before the formation of the heart, these organs begin to appear. They are at first small lamellar appen dices of extreme simplicity, attached above the three pairs of maxillary extremities, and repre senting the flabelliform portions of these limbs. Soon these lamellar appendages elongate and divide into two halv.es, one internal, lamel lar and triangular, the other external, small and cylindrical ; lastly, upon the surface of this, strive are observed to appear, which are the rudiments of the branchial filaments. During this interval the thoracic extremities have become developed, and above their bases other branchim have made their appearance, presenting in the beginning the form of tuber cles, and subsequently that of stilets; smooth and rounded on their surface, but by-and-by becoming covered vvith a multitude. of small tuberculations, which by their elongation are gradually converted into branchial filaments similar to the preceding. During this period of the development of the branchia2 these organs are applied like the extremities to the inferior surface of the embryo ; but they sub sequently rise against the lateral parts of the thorax, become lodged within a cavity situated under the carapace, and thus are no longer visible externally.