Chap. V.] 



HEART OF ARTHROPODA. 



In the fish (Fig. 82 D) the enlargement for the heart (H) is 

 found on the ventral vessel; passing forwardsit branches 

 on either side into' the branchial vessels, and these unite 

 and pour their blood into the dorsal (br) aorta (da). 



The blood-vascular system of the Arthropoda 

 is distinguished by the fact that the blood which 

 comes to it to be pumped through the body does not 



d 





Fig. 81. Diagrams to show the arrangement of the great Blood-vessels 

 in the Earthworm (A) ; the Crayfish (B). 



reach it directly by distinct vessels ; the heart is sur- 

 rounded by an imperfectly closed space, the 

 pericardial sinus. When the contractile cham- 

 ber which is called the heart dilates, the blood in the 

 surrounding sinus flows into it through two or more 

 spaces or holes in its walls. The heart may be short, 

 as in Daphnia, and have only a pair of orifices ; or it 

 may be greatly elongated, as in Artemia, where there 

 are twenty pairs ; or it may be much more compact, 

 as in the crayfish, where there are three pairs 

 of large ostia, one superior, one lateral, and one 

 inferior, \vhich are guarded by valves that prevent 



