THE CIRCULATION OF THE BLOOD 313 



As a rule, in all animals having any circulation at all, 

 the current always takes one direction. This is gener- 

 ally necessitated by valves. But a curious exception is 

 presented by the ascidians, whose tubular heart is valve- 

 less, and the contractions occur alternately at one end 

 and then the other ; so that the blood oscillates to and 

 fro, and a given vessel is at one time a vein and at an- 

 other an artery. In this respect it resembles the foetal 

 heart of higher animals (Fig. 364). 



In vertebrates only is the circulating 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 no great 

 advance in the apparatus 

 of the lowest vertebrates 

 over that of the highest 

 mollusks. A fish's heart 

 has, like that of an oyster, 

 two cavities, but its posi- 

 tion is reversed. Instead 

 of driving arterial blood 

 over the body, it receives 

 the returning, or venous, FlG 2&9 "_ Diagra V of a S in g ie Heart! <f, 



blood, and Sends it tO the auricle ' '> ven ficle; c, veins leading to 



auricle; a, aorta, or mam artery. 



gills. Recollected from the 



gills, the blood is passed into a large artery, or aorta, 

 along the back, which distributes it by a complex sys- 

 tem of capillaries among the tissues. These capillaries 

 unite with the ends of the veins which pass the blood 

 into the auricle 114 (Figs. 268, 272). 



In amphibians and in reptiles generally (as frogs, 

 snakes, lizards, and turtles), the heart has three cavities 

 two auricles and one ventricle. The venous blood 

 from the body is received into the right auricle, and the 



