364 



THE CRAYFISH 



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 

 sinuses into the great sternal sinus ; from the sternal sinus 

 by afferent branchial veins to the gills, where it exchanges 



Hhit St s 



Fig. 89. — Diajjr.im illustrating the course of the circulation of the blood in the 

 Crayfish. Heart and arteries red ; \eins and sinuses containing non-aerated 

 hlood 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 (/li) by a \-alvular 

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

 by valves (zr-.) ; the ultimate branches of the arteries discharge the blood into 

 sinia.ses (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 effert.nt branchial 

 veins {ef. by. r') into the branchiocardiac veins {br. c. t) which open into the 

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



carbon dio.xide 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, 

 or organ of propulsion : (2) a system of out-going channels, 

 the arteries, which carry the blood from the heart to the 



