THE CIRCULATION OF THE BLOOD. 



107 



A still higher form is seen in the Cnttle-fish, the high- 

 est of the Invertebrates. This animal has a central heart, 

 with a ventricle and two auricles, 

 and, in addition, the veins wliich "^s 

 collect tiie blood from the system 

 to send it back to the heart by 

 the way of the gills ai-e furnished 

 with two tranchial hearts, which 

 accelerate the circulation thi'dugh 

 those organs. Many of the arte- 

 ries and veins are joined by cap- 

 illaries, but not all ; so that in 

 no invertebrate animal is the 

 blood returned to the heart by a 

 continuousclosed system of blood- 

 vessels. 



As a rule, in all animals hav- 

 ing any circulation at all, the cur- 

 rent always takes one direction. 

 This is generally necessitated by 

 valves. But a curious exception 

 is presented by the Ascidians, 

 whose tubular heart is valveless, 

 and the contractions occur alter- 

 nately at one end and then the 

 other; so that the blood oscil- 

 lates to and fro, and a given ves- 

 sel is at one time a vein and at 

 another an artery. In this re- 

 jSpect it resembles the foetal heart 

 of higher animals (Fig. 279). 



In Yertebrates only is the cir- 

 culating current strictly confi'ned 

 to the blood-vessels ; in no case does it escape into the 

 general cavity of the body. In other respects, there is 



Pio. 71.— Circulating Apparatne in 

 the Fish : o, branchial ailery ; h, 

 arterial bnlb ; c, ventricle ; ^, au- 

 ricle ; fi, venous sinus ; /, portal 

 vein ; g, intestine ; A, vena cava ; 

 t*, branchial vessels ; k^ dorsal ar- 

 tery, or aorta; Z, kidneys; m, 

 dorsal artery. 



