The heart is always found in the median line of the body, and lying over the alimen tary canal near the dorsal aspect. Its form is various ; in the Decapods it is nearly square, and lies in the middle and superior part of the thorax, being separated from the carapace by tegumentary membranes only, and may be seen in the space included between the two vaults of the flancs. In structure it appears to be composed by the interlacement of nu merous muscular fibres, fixed by their extremi ties to neighbouring parts and passing to some distance over the aggregate at either end, so that the whole organ brings to mind such a figure as would be formed by the super position of a number of stars the rays of which do not correspond. In the other orders this general form of the heart varies conside rably, from the figure of an oblong square of rather inconsiderable size, as it occurs in the Decapoda (fig. 418,f), to that of a long cylin drical vessel extending through the whole Length of the body as it appears in the Stoma poda (fig. 419), and the Edriophthalmians. In the former of these it gives origin to six vascular trunks, three of which issue from the anterior edge and three from the posterior surface ; each' of the.six openings is closed by a valvular apparatus which prevents the regur gitation of the blood.
The first of the three anterior vessels is situated in the median line and is distributed to the eyes, in consequence of which We have entitled it the ophthalmic artery (a, fig. 418). Lodged within the substance of the general te gumentary membrane, it continues its course without undergoing any subdivision along the median line through the whole length of the thorax, until, arrived opposite the eyes, it sub divides and terminates in two branches vvhich penetrate the ocular peduncles.
On the two sides are the two antennary arteries. They run obliquely towards the an tennae, sending off numerous branches to the tegumentary membrane in which they are at first lodged; they then plunge more deeply, sending branches to the stomach and its mus cles and to the organs of generation, between which they insinuate themselves by following the folds of the same membrane which parts them. Lastly, each of these vessels subdivides into two branches, one of which proceeds to the internal and the other to the external antenna.
Two hepatic arteries arise from the fore part of the inferior surface of the heart, and pene trate the liver, there to be ramified ; but they are only found double and distinct from one another so long as the liver is met with divided into two lobes, as it is in the River-crab and Lobster.
From the posterior part of the same surface of the heart there proceeds a large trunk, which, from its importance, might be compared with the aorta. This is unquestionably the vessel
which many authors have spoken of as a great vena cava : we have entitled it the sternal artery. It bends forwards, giving origin to two abdo minal arteries (o,fig. 418), dips into the sternal canal, distributing branches to the different thoracic rings, as also to the five first eephalic rings, which it passes over in its course. Meet ing with the cesophagus it bifurcates, but still sends branches to the mandibles and the whole of the anterior and inferior parts of the head.
The bulb presented by the sternal artery at its origin, in the Macroura, is the part which Willis characterized as the auricle of the heart. As concerns the two abdominal arteries, which may be distinguished into superior and inferior, and which arise from the kind of cross which it forms almost immediately after its exit, they are in precise relationship in point of size with the magnitude and importance of the abdo men itself. In the Brachyura they are mere slender twigs; in the Macroura, on the con trary, they are capacious stems, and the inferior of the two sends branches to the two posterior pairs of thoracic extremities.
The disposition of the three first vessels is the same in the Stomapoda as in the preceding species ; but the great vessel which represents the heart being extended through the whole length of the body, supplies immediately other arterial branches in pairs, and in number equal to those of the rings.
The blood returns from the different parts of the body by canals, or rather vacuities among the tissues, (for they have no very evident appropriate parietes,) which terminate in the venous•sinuses, situated close to the branchi.
In the short-tailed Decapoda we find no more than a double series of these sinuses, included within the cells of the flancs above the articulations of the extremities. They com municate with one another, and they appear to have no parietes other than lamirm of cellular membrane of extreme tenuity which cover the neighbouring parts. Each of them, neverthe less, receives several venous conduits, and gives origin at its superior and external part to a vessel which, traversing the walls of the flancs at the base of the branchice, conducts the blood to the latter organs. This is the external or afferent vessel of the branchia2.
We find the same lateral venous sinuses in the Macroura; but instead of communicating with one another athwart the thoracic septa, as is the case in the Brachyura, they all empty themselves into a great median vessel, which is itself a venous sinus, and occupies the sternal canal. In the Squilla this sinus is al most the only vessel which serves as a reservoir to the venous blood.