THE WONDERS OF LIFE 



complex the organization of the higher animals, the 

 more necessary it is to have an orderly and regular dis- 

 tribution of the nutritive fluid to all parts. In the 

 coelenteria this work is accomplished by the gastric 

 canals (side branches from the gut, opening into its 

 cavity) but in the coelomaria it is done much better by 

 means of blood-vessels {vasa sanguijera) . These canals 

 do not communicate directly with the gastro-canal, but 

 are formed independently of it in the surrounding par- 

 enchyma of the mesoderm. They take up the filtered 

 and chemically improved food-fluid, which transudes 

 through the intestinal walls, and conduct it in the form 

 of blood to all parts of the body. This blood generally 

 contains millions of cells, which are of great importance 

 in metabolism. The blood-cells of the lower coelomaria 

 are usually colorless (leucocytes), while those of the 

 vertebrates are mostly red (rhodocytes). 



The circulation of the blood in most of the coelomaria 

 is effected by a heart, a contractile tube, formed by the 

 local thickening of a skin-vessel, which contracts and 

 beats regularly by means of its muscular bands. Origi- 

 nally two of these skin-vessels were developed in the 

 abdominal wall — a dorsal vessel in the upper and ven- 

 tral vessel in the lower wall (as in many of the ver- 

 malia). The heart is formed from the dorsal vessel in 

 the moUusks and articulates, but from the ventral in 

 the tunicates and vertebrates. The arteries are the 

 vessels which conduct the blood from the heart; those 

 which conduct it from the body to the heart are the 

 veins. The finest branchlets of both kinds of vessels, 

 which form the connecting link between them, are 

 called capillaries; these immediately effect the inter- 

 change of matter in the tissues by osmosis. The blood- 

 vessels co-operate very closely with the respiratory 

 organs. 



The interchange of gases in the organism, which we 



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