378 THE CRAYFISH CHAP. 



pressure of blood in the pericardia! 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 constantly 

 moving in one direction from the heart by the arteries 

 to the various organs of the body, where it receives 

 carbon dioxide and other waste matters ; thence by 



FIG. 96. Diagram illustrating the course of the circulation of the blood in the 

 Crayfish. Heart and arteries, red ; veins and sinuses containing non-aerated 

 blood, blue ; veins and sinuses containing aerated blood, pink. The arrows 

 show the direction of the flow. 



The blood from the pericardial sinus (pcd. s.) enters the heart (hi) by a valvular 

 aperture (v ') and is propelled into arteries (a), the orifices of which are guarded 

 by valves (u 2 ) ; the ultimate branches of the arteries discharge the blood into 

 sinuses (s), and the sinuses in various parts of the body debouch into the sternal 

 sinus (st. s.) ; thence the blood is taken by the afferent branchial veins (af. br. v) 

 into the gills, where it is purified and is returned by efferent branchial veins 

 (ef. br. v) into the branchiocardiac veins (br. c. v) which open into the pericardial 

 sinus. (From Parker and Haswell's Zoology.) 



sinuses into the great sternal sinus ; from the sternal 

 sinus by afferent branchial veins to the gills, where it 

 exchanges carbon dioxide for oxygen ; from the gills 

 by efferent branchial veins to the branchiocardiac veins, 

 thence into the pericardial sinus, and so to the heart 

 once more. 



It will be seen that the circulatory system of the 

 crayfish, like that of the frog, consists of three sections 

 (i) the heart, or organ of propulsion ; (2) a system of 



