340 THE CRAYFISH less. 



Comparing the blood-vessels of Astacus with those of 

 Polygordius, it would seem that the ophthalmic artery, 

 heart, and dorsal abdominal artery together answer to the 

 dorsal vessel, part of which has become enlarged and mus- 

 cular, and discharges the whole function of propelling the 



aJ.br V 



Fig. 88, — 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 i^pcd, s) enters the heart (///) by 

 a valvular aperture (»') and is propelled into arteries {a), the orifices of 

 which are guarded by valves (z/^) : the ultimate branches of the arteries 

 discharge the blood mto sinuses (j), and the sinuses in various parts of 

 the body debouch into the sternal sinus (j/. s) : thence the blood is taken 

 by the afferent branchial veins («/! br. v) into the gills, where it is purified 

 and is returned by efferent branchial veins (<?/! br. v) into the branchio- 

 cardiac veins (3r. c. v\ which open into the pericardial sinus. 



(From I'arker and Haswell's Zoology. ) 



blood. The horizontal portion of the sternal artery, together 

 with the ventral abdominal, represent the ventral vessel, 

 while the vertical portion of the sternal artery is a com- 

 missure, developed sometimes on the right, sometimes on 

 the left side, its fellow being suppressed. 



