COMPARATIVE PHYSIOLOGY. 325 



In the lowest animals (lowest Protozoa) we find that the 

 whole body (cell) serves as a digestive tract, and that food 

 can be taken in at any point on the surface. A little higher 

 (p. 322) an organ which we must call a mouth is formed in 

 the body, and this opening for the taking in (ingestion) of 

 food is found in all higher animals, except a few parasites 

 which, living in liquid food, need no such opening. With 

 larger animals a definite digestive cavity or canal is formed, 

 the lining of which has certain definite work to perform. 

 Most articles of food are insoluble as taken into the body ; 

 a bit of meat or starch can be soaked indefinitely in pure 

 water or can even be boiled for days without passing into 

 solution. In the digestive tract juices are produced which 

 alter these substances so that they can be dissolved ; and it 

 is only when they are in solution that they can pass through 

 the walls of the alimentary canal to those parts where they 

 are to be utilized. 



In the lower animals all parts of the digestive tract seem 

 able to act at once as formers of digestive fluids, and in the 

 taking up of the dissolved food, but as we pass higher in 

 the scale complications of various kinds are introduced. In 

 the first place, we find certain organs, like the salivary 

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

 tion of digestive fluids, and even in animals as low as the 

 sea-anemone the mesenterial filaments appear to have the 

 same power. On the other hand, the other portions, while 

 they may secrete, are pre-eminently the region for the 

 absorption of the liquefied food. Another complication is 

 this : A given amount of surface can absorb only so much 

 in a given time ; so 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 



