THE BLOOD OF ANIMALS. 97 
cast in Crustaceans; reddish, yellowish, or greenish, in 
Worms; and reddish, greenish, or brownish, in Jelly- 
fishes. The red liquid which appears when the head of 
a Fly is crushed is not blood, but comes from the eyes. 
In Fishes, Reptiles, Birds, and Mammals, the blood is red, 
excepting the white-blooded fish, Amphioxus.” 
As a rule, the more simple the fabric of the body, the 
more simple the nutritive fluid. In Sponges and Infu- 
soria, it is sea-water carrying organic particles; in the low 
Polyps and Jelly-fishes, it is merely chyme; in all other 
Invertebrates, having a complete alimentary canal, it is 
chyle; in the backboned animals, it is a highly complex 
and distinct fluid. 
In all animals, however, from Sponge to Man, the 
blood, apparently a clear, homogeneous fluid, really con- 
sists of minute grains, or globules, of organic matter float- 
ing in water. If the blood of a Frog be poured on a filter 
of blotting-paper, a 
transparent fluid (call- 
ed plasma) will pass 
through, leaving red 
particles, resembling 
sand, on the upper sur- 
face. Under the mi- 
croscope, these parti- 
cles prove to be cells, 
or flattened disks (call- 
ed corpuscles) contain- 
ing a nucleus 5 some 
A ] ‘| 4 Fra. 61.—Red Blood-corpuscles of Man: a, shows 
are COLO] ess, and oth- circular contour; 6b, a biconcave section; c¢, a 
ersred: Thered disks 2"? ™ chains 
have a tendency to run together; the colorless ones re- 
main single. Meanwhile, the plasma separates into two 
parts,by coagulating; that is, minute fibres form, consist- 
ing of fbrine, leaving a pale yellowish fluid, called se 
7 
