226 MANUAL OF ZOOLOGY sect 



place the afferent in communication with the efferent 

 branchial vein {ef. br. v), 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 



i)«t 



Fig 128. — Diagram of the circulation in the crayfish: heart and arteries, scarlet; 

 veins and sinuses containing non-aerated blood, blue: those containing aerated 

 blood, pink, <z, artery: af. br. z>, afferent branchial vein: br. c v, branchio- 

 cardiac vein: ef br v, efferent branchial vein; hi, heart; pcd s, pericardial 

 sinus; s, sinus; st. s, sternal sinus; v 1 . ostium with valves; v 2 , arterial valves. 

 The arrows show the direction of the current. 



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 the heart. 

 Thus in virtue of the successive contractions of the heart, 

 and of the disposition of the valves, the blood is kept con- 



