THE BLOOD OF ANIMALS. 97 



ards the heart. The valves of the lymphatics prevent its 

 return. 



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- 

 tishes. 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. 



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- 



7 



