VERTEBRATA. 



149 



the heart contains only bright, the other only dark, blood. 

 The division is also carried farther, for 

 the last arch (going to the lungs) becomes 

 connected with the half of the heart 

 which receives the dark blood, while 

 the rest of the arches are similarly 

 related to the other half of the heart. 



The blood itself should have a mo- 

 ment's attention. It consists of a fluid 

 (plasma) in which float myriads of 

 minute solid bodies (corpuscles). 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 cor- 

 puscles or leucocytes. The red corpuscles 

 carry the oxygen and carbon dioxide, 



FIG. 70 Diagram of the the plasma the nourishment and the 



circulation in a mam- 

 mal. The arrows show other waste. The plasma is further 

 the direction of the * 



flow; the vessels carry- p ecu ii ar i n that when withdrawn from 



ing red blood are r 



shown white, those the veins it soon solidifies or "clots." 



carrying dark blood, 



shaded, ^o, auricles; The excretory organs (kidneys or 

 K? rt v '^ Ringing nephridia) are very complicated struc- 



the blood from the in- * . ' J x 



testine ; v, ventricle, tures. 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 fluid the 

 nitrogenous waste and pours it into an excretory or urinary 

 duct which empties behind, near the anus. 



The reproductive system is closely related to the excretory 

 organs. In all except a few fishes the sexes are separate. 

 In the females, eggs are formed in special structures, the 



