378 THE CRAYFISH CHAP. 



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 

 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 



s 



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 (hf) by a valvular 

 aperture (z' 1 .) and is propelled into arteries (a), the orifices of which are guarded 

 by valves (z/ 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 

 (of. 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 peri- 

 cardial 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, 



