x PHYLUM ARTHROPODA 225 



(sf. 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. 118). Spaces in the gill-filaments 

 place the afferent in communication with the efferent 



sis 



FIG. 119. 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 ; of. br. i>. afferent branchial vein ; br. c, v. branchio- 

 cardiac vein ; ef. br. v. efferent branchial vein ; ht. heart ; pcd. s. pericardial 

 sinus ; j. sinus ; st. s* sternal sinus ; z/ 1 . ostium with valves ; z> 2 . arterial valves. 

 The arrows show the direction of the current. 



branchial vein (ef. br. #.), which occupies the inner side of 

 the gill-stem. The efferent branchial veins open into six 

 branchio-cardiac veins (br. c. v.\ which pass dorsally in close 

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

 pericardial 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, 



Man. Zool. 



