CHAPTER XV* 



THE BLOOD OF ANIMALS 



The Blood is that fluid which carries to the living 

 tissues 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, fishlike lancelet Amphioxus. m 



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 

 independent (as sponges), and in small and lowly organ- 

 ized animals (like hydra), there is no special circulating 

 fluid. Each cell feeds itself either directly from parti- 

 cles of food, or from the products of digestion. In 

 polyps and jellyfishes, the blood is scarcely different 

 from the products of digestion, although a few blood 

 corpuscles are present. But in the more highly organ- 

 ized invertebrates the blood is a distinct tissue, coagu- 



* See Appendix. 

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