314 



SYSTEMATIC ZOOLOGY. 



As has already been mentioned, in crocodiles, birds, and 

 mammals the ventricle is also divided, and hence one half 



of the heart contains only bright, 

 the other only dark, blood. The 

 division is also car ied farther, for 

 the last arch (going to the lungs) 

 becomes connected with the half of 

 the heart which receives the dark 

 b ood, while the rest of the arches 

 are similarly related to the other 

 half of the heart (fig. 118). 



The blood itself should have a 

 moment's attention. It consists of 

 a fluid (plasma) in which float myri- 

 ads of minute solid bodies (corpus- 

 cles). The plasma is a pale yellow 

 in color, the red of the blood being 

 due to certain of the corpuscles, 

 which are therefore known as the 

 red corpuscles. Other corpuscles 

 are colorless, and are called white 

 corpuscles or leucocytes. The red 

 corpuscles carry the oxygen and 

 carbon dioxide, the plasma the 

 nourishment and the other waste. 

 The plasma is further peculiar in 

 that when withdrawn from the veins it soon solidifies 

 or 'clots.' 



The excretory organs (kidneys or nephridia) are very 

 complicated structures. In a few words, they may be 

 described as a pair of organs lying in the dorsal wall of 

 the body-cavity close to the median line. Each kidney 

 is richly supplied with blood, and it extracts from this 



FIG. 118. Diagram of the 

 circulation in a mam- 

 mal. The arrows show 

 the direction of the flow ; 

 the vessels carrying red 

 blood are shown white, 

 those carrying dark 

 blood, shaded, a, au- 

 ricles; I, lung; Iv, liver; 

 p, portal vein bringing 

 the blood from the in- 

 testine; v, ventricle. 



