108 COMPAKATIVE ZOOLOGY. 



no great advance in the apparatus of the lowest Verte- 

 brates over that of the liighest Molhisks. A Fish's heart 



lias, like tliat of an Ovster, 

 two cavities, but its position 

 is reversed. Instead of driv- 

 ing arterial blood over the 

 body, it receives the return- 

 ing, or venous, blood, and 

 sends it to the gills. Ke- 

 collected from the gills, the 

 blood is passed into a large 

 artery, or aorta, along the 

 back, wiiich distributes it by 

 a complex system of capil- 



Fr&. 72 Diagnim of a single Heart: d, ■. . ,-, 



auricle; e, ventricle ; c, veins leading to iarieS among the tlSSUeS. 

 anricle ; a, aorta, or main artery. rp^^^^^ capillaries Unite witll 



the ends of the veins which pass the blood into the auri- 

 cle =" (Figs. 71, 75). 



In Amphibians and in Reptiles generally (as Frogs, 

 Snakes, Lizards, and Turtles), tiie heart has three cavities 

 — two auricles and one ventricle. Tlie venous blood from 

 tlie body is received into the right auricle, and the purified 

 blood from the lungs into tlie left. Both tiirow their con- 

 tents into the ventricle, which pumps the mixed blood in 

 two directions — partly to the lungs, and partly around the 

 system (Fig. 76). Circulation is, therefore, incomplete, since 

 the whole current does not pass through the lungs, and 

 three kinds of blood are found in the body — arterial, ve- 

 n.ous, and mixed. In many animals, however, arrange- 

 ments exist which nearly separate the venous from the 

 arterial blood. 



The ventricle of Eeptiles is partially divided by a par- 

 tition. In the Crocodile, the division is complete, so that 

 there are really four cavities — two auricles, and two ven- 

 tricles. But both ventricles send off aortas which cross 



