THE BLOOD OF ANIMALS. 
09 
there is little fibrin, and, therefore, it coagulates feebly or 
not at all. In the Arthropods and higher Mollusks, the 
circulating fluid contains 
colorless nucleated cells, 
and coagulates. 66 In Ver¬ 
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, 
Reptiles, and Birds, i. e.> 
all oviparous Vertebrates, 
these red corpuscles are Fia.63.-NucleatedBlood-cell 8 ofaFrog,x 2 B 0 . 
nucleated; but in those of Mammals, no nucleus has been 
discovered. 57 
All blood-corpuscles are microscopic. The colorless are 
more uniform in size than the red; and generally smaller 
(except in Mammals), being about 
Two- of an inch in diameter. The 
red corpuscles are largest in Amphib¬ 
ians (those of Proteus being the ex¬ 
treme, or -r^ 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 
awhUeprominfnceauhf with the size of the animal only with- 
centre * 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^Vir 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- 
