SUMMARY 173 



tions as we have seen, and we may summarize its functions as 

 follows : 



(a) It carries to the tissues the food which has been prepared 

 by digestion, and introduced into the plasma by absorption. 



(6) It carries oxygen, which it takes from the air in the lungs, 

 to the tissues and there exchanges it for carbon dioxide which it 

 carries away. The red corpuscles are the carriers in this case. 



(c) It carries from the tissues to the lungs, kidneys, and skin, 

 the wastes of the body. Such- wastes include carbon dioxide, 

 urea, acids, etc., and are removed from the body by the organs 

 mentioned. 



(d) It is the medium for the transportation of the internal 

 secretions of certain glands. 



(e) It distributes the heat produced in various parts of the 

 body and thus tends to equalize the body temperature. 



Blood in other animals. — Since the function of the blood is the 

 distribution of food and oxygen, and the collection of wastes, it is 

 evident that animals composed of a single or few cells will have 

 no need of such a fluid. In animals like the amoeba, and even 

 in animals of more complicated cellular structure, such as the 

 hydra, sponges, and jellyfishes, the digested food and the air are 

 brought in direct contact with the cells, and hence in these animals 

 there is no blood and no need of such fluid. As animals increase 

 in complexity, some of their cells are necessarily so placed that the 

 food is no longer brought in direct contact with them. In such 

 animals some system of distribution becomes necessary, and hence 

 most animals higher than those mentioned have some kind of fluid 

 that answers the purpose of the blood in our bodies. In many 

 this blood fluid is not inclosed in tubes and is without corpuscles. 

 It is to be compared rather with the plasma than with the red 

 blood of man. Crayfishes, crabs, and insects have such a fluid, 

 and it is colorless. In the earthworm the blood fluid is red, but 

 has no red corpuscles, the hasmoglobin being dissolved in the fluid 

 itself. It is only when we reach the vertebrate or backboned 

 animals that we find blood containing corpuscles specialized for the 

 carrying of oxygen and carbon dioxide. In the lower forms of 

 vertebrates these corpuscles are nucleated. Only in birds and 

 mammals do we find the non-nucleated corpuscle. 



