540 ZOOLOGY 



SECT. 



to the legs, jaws, &c. At the point where the sternal artery turns 

 forwards it gives off the median ventral abdominal artery (uaa.), 

 which passes backwards beneath the nerve-cord, and supplies the 

 ventral muscles, pleopods, &c. 



All these arteries branch extensively in the various organs they 

 supply, becoming divided into smaller and smaller offshoots, which 

 finally end in microscopic vessels called capillaries. These latter 

 end by open mouths which communicate with the blood-sinuses 

 (Fig. 447, s.), spacious cavities lying among the muscles and viscera, 



V2 ' P cd 



aJ.br v 



st s 



FIG. 447. Diagram of the circulation in the Crayfish; heart and arteries scarlet, veins and 

 sinuses containing non-aerated blood, blue ; those containing aerated blood, pink. a. artery ; 

 af.br.v. afferent branchial vein ; br.c.v. branchip-cardiac vein ; ef.br.v. efferent branchial 

 vein ; ht. heart ; pcd.s. pericardial sinus ; s. sinus ; st.s. sternal sinus ; ri. ostium with 

 valves ; t> 2 . arterial valves. The arrows show the direction of the current. 



and all communicating, mediately or immediately, with the sternal 

 simis (st.s.), a great median canal running longitudinally along the 

 thorax and abdomen, and containing the ventral nerve-cord and 

 the sternal and ventral abdominal arteries. In the thorax the 

 sternal sinus sends an offshoot to each gill in the form of a 

 well-defined vessel, which passes up the outer side of the gill and 

 is called the afferent branchial vein (af.br.v. ; see also Fig. 446). 

 Spaces in the gill-filaments place the afferent in communication 

 with the efferent branchial vein (ef.br.v.), which occupies the inner 

 side of the gill-stem. The eighteen efferent branchial veins open 

 into six branchiocardiac veins (br.c.v.), which pass dorsally in close 

 contact with the lateral wall of the thorax and open into the peri- 

 cardial sinus (pcd.s.). 



The whole of this system of cavities is full of blood, and the 

 heart is rhythmically contractile. When it contracts, the blood 

 contained in it is prevented from entering the pericardial sinus by 

 the closure of the valves of the ostia, and therefore takes the only 

 other course open to it, viz., into the arteries. When the heart 

 relaxes, the blood in the arteries is prevented from regurgitating 

 by the valves at their origins, and the pressure of blood in the 

 pericardial sinus forces open the valves of the ostia and so fills 



