THE BLOOD OF ANIMALS 



303 



FIG. 260. Nucleated Blood Cells of a Frog, x 

 250: a, colorless corpuscles. 



If now we examine the nutritive fluid pi the simplest 

 animals, we find only a watery fluid containing granules. 

 In radiates and the 

 worms and mollusks, 

 there is a similar fluid, 

 with the addition of 

 a few colorless corpus- 

 cles. But there is little 

 fibrin, and, therefore, 

 it coagulates feebly 

 or not at all. In the 

 arthropods and higher 

 mollusks, the circulat- 

 ing fluid contains col- 

 orless nucleated cells, 

 and coagulates. 110 In 

 vertebrates, there are, 



in addition to the plasma and colorless corpuscles of 

 invertebrates, red corpuscles, to which their blood owes 

 its peculiar hue. In fishes, amphibians, reptiles, and 

 birds, i.e., all oviparous vertebrates, these red corpuscles 

 are nucleated ; but in those of 

 mammals, no nucleus has been 

 discovered. 111 



All blood corpuscles are micro- 

 scopic. The colorless are more 

 uniform in size than the red ; and 

 generally smaller (except in mam- 

 mals), being about ^rVo f an mcn 

 in diameter. The red corpuscles 

 are largest in amphibians (those of 

 Proteus being the extreme, or ^o" 

 of an inch), next in fishes, then birds and mammals. 

 The smallest known are those of the musk deer. In 

 mammals, the size agrees with the size of the animal 



FIG. 261. Elliptical Corpuscle 

 of the Frog, showing the nu- 

 cleus as a prominence in the 

 center. Greatly magnified. 



