204 



ANIMALS 



Vertebrates. In the cockroach the sahvary glands have an 

 amylolytic action. The intestinal fluids have amylolytic, pro- 

 teolytic and inverting action^ and the reaction is neutral and 

 alkaline. 



468. Circulation. — However simple or complicated the diges- 

 tive processes may be, the result is essentially the same. The 

 end finally attained is food substances prepared for absorption. 

 This is a function entirely distinct from digestion, and since 



each cell must absorb food for itself, 

 little differentiation is to be looked for 

 in connection with this function. How- 

 ever, only those cells can absorb which 

 are in contact with the food, i. e., the 

 cells lining the digestive tract. Those 

 farther removed must receive their por- 

 tion from those nearer the source. In 

 hydra no cell is more than one cell re- 

 moved from the seat of digestion, for 

 the gastro- vascular cavity extends to 

 , , , all parts of the body, even to the tips 



and two sets of slender mus- ^ i i 1 t * 



cle fibres crossing each other of the tentacles, and whether digestion 

 at right angles. takes place in the gastro-vascular cavity 



or in the entodermal cells the ectoderm cells are only one cell 

 layer removed. 



469. In the smallest Annelids the intestinal wall is very thin. 

 The same is true of the body wall, and the two are separated by 

 a space, the body cavity, which is filled with a fluid (''body 

 fluid"). This body fluid nourishes the tissues bathed by it 

 and it is constantly replenished by the substances absorbed by 

 the intestine. The movements of the animal force the body 

 fluid about so that it becomes throughly mixed and freshly 

 absorbed matter is thus directly brought to the farthest tissues 

 of the body. 



470. In the larger worms, however, the tissues are often so 



Fig. 112. — A view of the 

 outer surface of the intes- 

 tine of nereis, showing a 

 network of blood capillaries 



