THE CIRCULATION OF THE BLOOD. 



107 



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

 est of the Invertebrates. This animal lias 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 

 accelerate the circulation through 

 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 

 contin nous closed 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- 

 spect it resembles the foetal heart 

 of higher animals (Fig. 279). 



In Vertebrates only is the cir- 

 culating current strictly confined 

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

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



h- 



PIG. 71. Circulating Apparatus in 

 the Fish : o, branchial artery ; &, 

 arterial bulb; c, ventricle ; d, au- 

 ricle ; e, venous sinus ; /, portal 

 vein ; 7, intestine ; A, vena cava ; 

 t, branchial vessels ; k, dorsal ar- 

 tery, or aorta; I, kidneys; m, 

 dorsal artery. 



