THE PHYSIOLOGY OF NUTRITION. 741 



to find clear indications of the presence of ferments in the 

 gut. He inclined to the opinion that in many members of 

 the class, the digestive processes are as simple as those of 

 the Ccelentera. 



The Nutrition of the Tissues. 



After the complex food substances have been broken 

 down into simpler ones, they must be carried to the tissues, 

 there to be employed in repairing waste, or in growth. In 

 a simple Protozoon there is no difficulty ; like a primitive 

 community, the single cell supplies its own wants, and the 

 question of transport is never raised. In a Metazoon, on 

 the other hand, as in a civilised state, there is much 

 division of labour, and the question of the transport of 

 manufactured material becomes very important. 



In a Vertebrate the blood is the great transporting 

 agent ; into it the products of digestion are ultimately 

 poured ; from it waste products are filtered. It is itself, 

 however, confined to closed vessels, and does not come 

 into close connecton with the tissues ; these are, strictly 

 speaking, nourished by the lymph, which bathes the tissues 

 throughout, and also communicates freely with the blood 

 stream. Thus the lymph is the " middleman " between 

 blood and tissues. In Vertebrates the lymph has not the 

 respiratory significance which the blood has in virtue of its 

 red corpuscles. 



In most of the lower aquatic forms of life, the fluid 

 within the body differs little from that which surrounds it. 

 Thus, as we should expect, the fluid which bathes the cavity 

 of a sea anemone or a jellyfish, filling the hollow tentacles 

 of the one and the canal system of the other, is little more 

 than sea water. It contains no formed elements, no dis- 

 solved albumens, no organic substances capable of forming 

 a loose combination with oxygen that is, no respiratory 

 pigment. It is thus certainly not a nutritive fluid ; the tissues 

 must be nourished by the products of digestion passing 

 from cell to cell. It is, however, of use in respiration. Like 

 other sea water, it contains dissolved oxygen ; and we must 

 suppose that the endoderm cells take up the oxygen they 

 require directly from it, as the ectoderm cells do from the 

 surrounding water. The fluid has also an excretory signi- 



