The heart (fig. 15, _h_), lies in the midst of this sinus. It is a thick muscular body (fig. 16), with an irregularly hexagonal contour when viewed from above, one angle of the hexagon being anterior and another posterior. The lateral angles of the hexagon are connected by bands of fibrous tissue (_ac_) with the walls of the pericardial sinus. Otherwise, the heart is free, except in so far as it is kept in place by the arteries which leave it and traverse the walls of the pericardium. One of these arteries (figs. 5, 12, and 16, _saa_), starting from the hinder part of the heart, of which it is a sort of continuation, runs along the middle line of the abdomen above the intestine, to which it gives off many branches. A second large artery starts from a dilatation, which is common to it with the foregoing, but passing directly downwards (figs. 12 and 15, _sa_, and fig. 16, _st.a_), either on the right or on the left side of the intestine, {72} traverses the nervous cord (figs. 12 and 15), and divides into an anterior (fig. 12, _sa_) and a posterior (_iaa_) branch, both of which run beneath and parallel with that cord. A third artery runs, from the front part of the heart, forwards in the middle line, over the stomach, to the eyes and fore part of the head (figs. 5, 12, and 16, _oa_); and two others diverge one on each side of this, and sweep {73} round the stomach to the antennæ (_aa_). Behind these, yet two other arteries are given off from the under side of the heart, and supply the liver (_ha_). All these arteries branch out and eventually terminate in fine, so-called _capillary_, ramifications. [Illustration: FIG. 16.—_Astacus fluviatilis._—The heart (× 4). A, from above; B, from below; C, from the left side. _aa_, antennary artery; _ac_, alæ cordis, or fibrous bands connecting the heart with the walls of the pericardial sinus; _b_, bulbous dilatation at the origin of the sternal artery; _ha_, hepatic artery; _la_, lateral valvular apertures; _oa_, ophthalmic artery; _s.a_, superior valvular apertures; _s.a.a_, superior abdominal artery; _st.a_, sternal artery, in B cut off close to its origin.] In the dorsal wall of the heart two small oval apertures are visible, provided with valvular lips (fig. 16, _sa_), which open inwards, or towards the internal cavity of the heart. There is a similar aperture in each of the lateral faces of the heart (_la_), and two others in its inferior face (_ia_), making six in all. These apertures readily admit fluid into the heart, but oppose its exit. On the other hand, at the origins of the arteries, there are small valvular folds, directed in such a manner as to permit the exit of fluid from the heart, while they prevent its entrance.