PHYSIOLOGY. 397 



glands, stomach, pancreas, and liver, set apart for the 

 secretion of digestive fluids, and even in animals as low 

 as the sea-anemone the mesenterial filaments (p. 169) 

 appear to have the same power. On the other hand, the 

 other portions, while they may secrete, are pre-eminently 

 the regions for the absorption of the liquefied food. An- 

 other complication is this : A given amount of surface can 

 absorb only so much in a given time; so as to obtain the 

 necessary amount of food the surface must be increased. 

 This explains in part the folding of the wall of the digestive 

 tract in the sea-anemone, as well as the lengthening and 

 coiling of the intestine in tadpole and rat, and the spiral 

 valve in the shark. In manv vertebrates the surface is 



*/ 



still further increased by numerous minute foldings and 

 outpushings of the lining of the intestine which, though 

 so small as to be invisible to the naked eye, still more than 

 double the surface. 



With most food there are certain portions which are 

 indigestible. These of course must be eliminated. In 

 the Coelenterates and flatworms the only opening through 

 which they can pass out is the same one by which they 

 entered, and so this opening, usually called the mouth, 

 serves at once as mouth and vent. In the higher forms 

 the alimentary canal becomes a complete tube with two 

 distinct openings, one the mouth for the taking in of 

 food, the other anus or vent for the ejection of non- 

 nutritious portions. 



After its solution the food (nourishment) must be trans- 

 ferred to the parts which are to do the work. In the 

 Protozoa the same parts which digest do the work. In 

 the sea-anemone and flatworms the pouching of the diges- 

 tive tract (figs. 17, 24) renders the transfer easy, for the 

 pouches extend to all parts. Above these forms we find 



