THE HIGHER ORGANISMS 135 



time, becomes more concentrated and as its specific 

 gravity increases its oxygen-absorbing capacity is also 

 increased. 



With the appearance of true respiratory organs 

 gills in the Crustacea the circulation becomes modified 

 in a simple fashion. The primitive heart discharges 

 the blood into several large vessels one of which conveys 

 it principally to the gills, where it is aerated as will be 

 shown later, from which it returns to the heart to become 

 mixed with the blood returning through other afferent 

 vessels, imparting its oxygen to the whole with which 

 it mixes before being sent upon a new circuit. The 

 aerating circuit is not definitely separated from that of 

 the tissues in the neighborhood of the gills; a very small 

 fraction of the total blood is carried to the gills and 

 after being aerated it mixes with the general blood mass. 

 The arrangement is very imperfect when contrasted with 

 that found in mammals, but answers the necessities of 

 the animals in which it occurs. 



Among some of the invertebrates the blood corpuscles 

 are found to contain small quantities of a reddish sub- 

 stance known as hemoglobin which forms a loose combi- 

 nation with oxygen highly advantageous to the blood 

 by increasing its oxygen-carrying power. Among the 

 vertebrates, however, the blood corpuscles are always 

 of two kinds, the whites or leucocytes which are amoeboid 

 and contain no hemoglobin, and the reds or erythrocytes 

 which invariably contain it, the proportion of the latter 

 increasing until among the highest mammals they exceed 

 the leucocytes in the proportion of 1 white to 750 red. 

 At first the corpuscles scarcely differ from one another 

 except in containing hemoglobin, but eventually the 

 erythrocytes become so differentiated that they appear 

 only as minute discs or cups of hyaline stroma without 

 nuclei and thoroughly impregnated with hemoglobin. 

 This enables the blood to absorb and transport to the 

 tissues immensely more oxygen than would otherwise 

 be possible. 



