68 ZOOLOGY 



and lymph; and its transportation in these systems, which is 

 made necessary 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 bene- 

 fit 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 (oxidation; 

 see §26) 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 elimi- 

 nated 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. Most so- 

 called secretions are really excretions from the point of view of 

 the protoplasm which produces them. 



93. The Digestive System. — The simplest condition of 

 the digestive tract is found in the gastrula (archenteron. Fig. 

 13, 4) or in Hydra (Fig. 81). 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. 31. A still more complicated condition is shown in 

 Fig. 95. In this form 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 caslom 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 (proctodcsum) also lined with ectoderm, and 

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

 stomodaeum 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 

 secrete materials that act upon foods in such a way as to 

 render them capable of being absorbed through the entodermic 

 cells into the body cavity, or into that special portion of it known 



