50 APPLIED BIOLOGY 



Absorption. If the frog were a small animal with all 

 its tissues near the food-tube or digestive-cavity (i.e., the 

 stomach and intestine) , the dissolved food could be absorbed 

 through the walls of the food-tube into the surrounding 

 organs. This is the case in some very small animals of 

 simple structure, which will be studied later. We can il- 

 lustrate the absorption of digested (liquid) food in such a 

 simple animal by the following experiment : 



(Z)) Take four sheets of blotting-paper (or filter-paper) about 4 

 by 10 inches in size, place them together, then roll them into a cone, 

 and fasten with a pin. Dissolve common salt in water to make a 

 strong solution, and pour into the cavity of the paper cone. Notice 

 that the salt solution is absorbed first by the inner layer of paper, 

 and then in succession by the outer layers, each layer absorbing 

 from the next one nearer the center. Now unroll the sheets, 

 mark the inner one No. 1 and the outer No. 4, and dry them. 

 When dry notice the crystals of salt on sheet No. 4, proving that the 

 salt in solution has been absorbed from sheet No 1 to 2, from 2 to 

 3, and from 3 to 4. 



Some simple animals have a cylindrical body made up of 

 several layers of cells which are in close contact similar to 

 that of the layers of paper in the above experiment. (See 

 Fig. 98, of Hydra.) In such simple animals the cavity in the 

 center of the body is the digestive cavity in which food is 

 prepared for absorption. The cells which line this cavity 

 are in contact with dissolved food and can absorb directly, 

 just as sheet No 1 did in the above experiment ; and the 

 cells which form the outer layers must absorb from the inner 

 ones, just as in the above experiment the outer layers of 

 paper absorbed from those nearer the center. 



46. Need of Blood for Distributing Food. Absorption 

 directly from the digestive cavity of the stomach or intestine 

 or both would be possible only in a very simple animal. If 

 in the above experiment we had used twenty or thirty sheets 

 of paper, we should have found that very little salt solution 

 would soak through to the outermost layer ; and likewise in 



