THE BLOOD OF ANIMALS. 97 
the heart. The valves of the lymphatics prevent its re¬ 
turn. 
CHAPTER XII* 
THE BLOOD OF ANIMALS. 
The Blood is that fluid which carries to the living tis¬ 
sues the materials necessary to their growth and repair, 
and removes their waste and worn-out material. The 
great bulk of the body is occupied with apparatus for the 
preparation and circulation of this vital fluid. 
The blood of the lower animals (Invertebrates) differs 
so widely from that of Man and other Vertebrates, that 
the former were long supposed to be without blood. In 
them the blood is commonly colorless; but it has a bluish 
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 Vertebrates, the blood is red, excepting the white- 
blooded fish, Amphioxus , 64 
As a rule, the more simple the fabric of the body, the 
more simple the nutritive fluid. In unicellular animals 
(as Protozoa), in those whose cells are comparatively inde¬ 
pendent (as Sponges), and in small and lowly organized 
animals (like Hydra), there is no special circulating fluid. 
Each cell feeds itself either directly from particles of 
food, or from the products of digestion. In Polyps and 
Jelly-fishes, the blood is scarcely different from the prod¬ 
ucts of digestion, although a few blood-corpuscles are pres¬ 
ent. But in the more highly organized Invertebrates the 
blood is a distinct tissue, coagulating, and containing 
white corpuscles. The blood of the Vertebrates, appar- 
* See Appendix- 
7 
