312 



COMPARATIVE ZOOLOGY 



and a ventricle, one for receiving, and the other for 

 distributing, the blood. The auricle injects the blood 

 into the ventricle, which propels it by the arteries to 

 the various organs. Thence it passes not immediately 



to the veins, as in higher ani- 

 mals, but into the spaces around 

 the alimentary canal. A part 

 of this is carried by vessels to 

 the gills or lung, and then re- 

 turned with the unpurified por- 

 tion to the auricle. The whole 

 of the blood, therefore, does 

 not make a complete circuit. 

 The clam has a similar heart, 

 but with two auricles. 



A still higher form is seen 

 in the cuttlefish, the highest of 

 the invertebrates. This animal 

 has a central heart, with a 

 ventricle and two auricles, 

 and, in addition, the veins 

 which collect the blood from 

 the system to send it back to 

 the heart by the way of the 

 gills are furnished with two 

 branchial hearts, which accel- 

 erate the circulation through 

 those organs. Many of the 

 arteries and veins are joined 

 by capillaries, but not all ; 

 so that in no invertebrate 



animai is the bi d returned 



ricle; e, venous sinus; /, portal to ^ Q heart by a COntinUOUS 



vein; g, intestine; h, vena cava; J 



f, branchial vessels; k, dorsal ar- closed System of blood VCS- 



tery, or aorta; I, kidneys; m, > 



dorsal artery. S61S. 



h~ 



FIG 268. circulating Apparatus in 



