THE GENERAL ANIMAL FUNCTIONS. 6l 



cooperation of numerous organs. It embraces the ingestion 

 or taking in of food; the digestion of food; its absorption 

 from the digestive tract into the body fluids blood and lymph ; 

 and its transportation in these systems, which is made neces- 

 sary by the fact that digestion is confined to a special region. 

 It likewise includes the further absorption of these materials 

 from the blood and lymph by the cells for whose benefit all 

 the preceding work has been done; the assimilative process 

 within the cell whereby the food material is made into proto- 

 plasm or other complex cell-products ; the reception and trans- 

 mission of oxygen, by the combining power of which (oxida- 

 tion; see 25) these complex substances are broken down 

 into simpler ones useful, useless,, or hurtful to the animal 

 economy. Finally, the elimination of the products of this 

 oxidation or burning is a necessary part of the nutritive 

 process. If the material eliminated from the cell is of further 

 use the process is known as secretion, if not, excretion. It is 

 undesirable to attempt to make a sharp distinction between 

 excretion and secretion. 



90. The Digestive System. The simplest condition of 

 the digestive tract is found in the gastrula (arehenteron, Fig. 

 n, 4) or in Hydra (Fig. 79). Here there is only one cavity 

 in the body and the food is taken up immediately by the cells 

 needing it. A simple modification of this condition is seen 

 in Fig. 29. A still more complicated condition is shown in 

 Fig- 93- I* 1 tms f rm which we may take as the type, the 

 digestive tract is a tube, running through the body, lined with 

 its own epithelium and is separated from the body wall by 

 the cocloni or body cavity. The tube itself may have any 

 degree of complexity, but consists essentially of (i) an an- 

 terior portion (stomodceum) lined with ectoderm, (2) a pos- 

 terior portion (proctodoeum) also lined with ectoderm, and 

 (3) a middle portion (mesenteron) lined with entoderm. The 

 stomodceum or mouth region is usually supplied with devices 

 for the capture and ingestion of the food. The mesenteron 

 is the true digestive region. It is supplied with cells which 



