THE BLOOD OF ANIMALS. 



99 



there is little fibrine, and, therefore, it coagulates feebly or 

 not at all. In the Arthropods and higher Mollusks, the 

 circulating fluid contains 

 colorless nucleated cells, 

 and coagulates. 56 InVer- 

 tebrates, there are, in ad- 

 dition to the plasma and 

 white corpuscles of In- 

 vertebrates, red corpus- 

 cles, to which their blood 

 owes its peculiar hue. 

 In Fishes, Amphibians, 

 Keptiles, and Birds, i. e., 

 all oviparous Vertebrates, 



these red COrpUSCleS are Fio.63.-Nu C leatedBlood-cell S ofaFrog,x 250. 



nucleated; but in those of Mammals, no nucleus has been 



discovered. 57 



All blood -corpuscles are microscopic. The white are 



more uniform in size than the red ; and generally smaller 

 (except in Mammals), being about 

 Winr f an i ncn * n diameter. The 

 red corpuscles are largest in Amphib- 

 ians (those of Proteus being the ex- 

 treme, or T | of an inch), next in 

 Fishes, then Birds and Mammals. The 

 smallest known are those of the Musk- 



FIG. 64. -Elliptical Corpus- deer. In Mammals, the size agrees 



cle of the Frog, showing ..v,i . /. ,1 i , 



a white prominence at the with the size of the animal only with- 

 in a natural order; but in Birds the 

 correspondence holds good throughout the class, the larg- 

 est being found in the Ostrich, and the smallest in the 

 Humming-bird. In Man, they measure -g-^VTr of an inch, 

 so that it would take 40,000 to cover the head of a 

 pin. 



As to shape, the colorless corpuscles are ordinarily glob- 



