CHAPTER IX 

 THE BLOOD AND ITS FUNCTIONS 



Blood and blood vessels are entirely lacking in some of the 

 lower animals, as for example, in sponges, jelly fishes, corals 

 and the lowest of the worms, the so-called "flat-worms." In 

 the insects, too, the blood system is present only in a weak, 

 poorly formed way. This seems particularly strange when 

 one considers how dependent a human being is on the blood 

 system; so dependent, indeed, that after a severe cut a person 

 may die from loss of blood. Blood, in fact, is considered the 

 symbol, if not the synonym, of life itself. 



By studying these animals, however, we find the reason 

 why they do not need blood systems. The sponge has numer- 

 ous pores opening into the body; these lead to canals which 

 extend through the substance of the animal, thus carrying all 

 over the body the water and food substances, which enter 

 the canals. At the same time the sponge obtains oxygen from 

 the water circulating through the canals. The jelly-fishes, 

 corals and flat-worms .all have complex stomach cavities; 

 numerous pouches and ducts leading away from the stomach 

 carry the food particles to all parts of their bodies. So much 

 water is taken in with their food that these animals obtain 

 all the oxygen they need from the same water in which their 

 food floats. Insects have a tubular digestive tract going 

 almost straight through the animal; but besides this there is 

 a great network of air tubes all over the body, between the 

 muscles, passing into the legs, wings and, in fact, everywhere. 

 These bring air in through sets of pores in the "skin/ 7 and 

 take it over the body. In the human being the digestive tract 

 is a tube, with no side branches of any sort; and all the air one 

 in goes into two comparatively small sacs, the lungs. 

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