[Illustration: FIG. 15.—_Astacus fluviatilis._—A diagrammatic transverse section of the thorax through the twelfth somite, showing the course of the circulation of the blood (× 3). _arb. 12_, the anterior or lower, and _arb′. 12_, the posterior or upper arthrobranchia of the twelfth somite; _av_, afferent branchial vessel; _bcv_, branchio-cardiac vein; _bg_, branchiostegite; _em_, extensor muscles of abdomen; _ep_, epimeral wall of thoracic cavity; _ev_, efferent branchial vessel; _fm_, flexor muscles of abdomen; _fp_, floor of pericardium; _gn._ 6, fifth thoracic ganglion; _h_, heart; _hg_, hind-gut; _iaa_, inferior abdominal artery, in cross section; _la_, lateral valvular apertures of heart; _lr_, liver; _mp_, indicates the position of the mesophragm by which the sternal canal is bounded laterally; _p_, pericardial sinus; _pdb. 12_, podobranchia, and _plb. 12_, pleurobranchia of the twelfth somite; _sa_, sternal artery; _saa_, superior abdominal artery; _sc_, sternal canal; _t_, testis; XII., sternum of twelfth somite. The arrows indicate the direction of the blood flow.] The blood of the crayfish is kept in a state of constant circulating motion by a pumping and distributing machinery, composed of the _heart_ and of the _arteries_, with {71} their larger and smaller branches, which proceed from it and ramify through the body, to terminate eventually in the blood sinuses, which represent the veins of the higher animals. When the carapace is removed from the middle of the region which lies behind the cervical groove, that is, when the dorsal or _tergal_ wall of the thorax is taken away, a spacious chamber is laid open which is full of blood. This is the cavity already mentioned as the _pericardium_ (fig. 15, _p_), though, as it differs in some respects from that which is so named in the higher animals, it will be better to term it the _pericardial sinus_.