204 



OF THE BLOOD AtfD CIBCULATIOX. 



Auricle. 



Ventricle. 



Veins. 



Heart. 



Veins. 



lying along the spine, and by its branches is distributed through- 

 out the body. Fig. 220 is a plan of this type of circulating 

 organ. 



Lesser circulation. [ 368. In the 



mollusca the heart 

 consists of a ven- 

 tricle and an au- 

 ricle, as in fishes ; 

 but it differs in 

 this, that it is 

 destined to pro- 

 pel the blood 

 through the sys- 



^ Dorsal artery. teni) an( J not 



through the gills, 

 as in that class. 



[Fig. 221 repre- 

 sents the circula- 

 ting organs of the 

 Doris; the heart 

 consists of a ven- 

 tricle (a), from 

 whence arises the 

 aorta (6), which sends branches to all parts of the body ; and a 

 single or double auricle (c), in which the veins (d) of the bran- 

 chial organs (e) terminate, the branchiae being developed in the 

 form of external vascular tufts. The blood purified in these or- 

 gans is conveyed to the heart, and transmitted by arteries through 

 the body ; it is collected by the radicles of the veins, which 

 terminate in a large trunk (f) . By this vena cava it is dis- 

 tributed through the gills (<?), and from these organs it is re- 

 turned to the heart. In the CEPHALOPODA the circulation 

 through the gills is aided by branchial ventricles, situated at 

 the bases of these organs, but in other respects their circu- 

 latory apparatus resembles that of the mollusca in general. 



[ 369. In the Crustacea (fig. 222), the circulation is after 

 the type of the mollusca. The heart (a) consists of a ven- 

 tricle only, from which several arteries arise ; the opthalmic 

 (b), the antennal(c), the hepatic (d) t the superior abdominal (e), 

 and the sternal (/). After having circulated through the body, 

 the blood is collected in certain reservoirs (g ff), which take the 



Greater circulation. 

 Fig. 220. Circulation in fishes. 



