THE SIMPLEST MANY-CELLED ANIMALS 327 



amoeba-like and able to take in food in the way that an 

 amoeba does. Particles thus taken into the cells are com- 

 pletely digested (dissolved). This is intra-cellular digestion 

 (i.e., within cells), and is the same as the only possible 

 digestion in Protozoa and Porifera. But in Hydra there is 

 a decided advance over the lower animals in that the cells 

 able to digest intra-cellularly are grouped so as to form the 

 lining of a cavity (digestive cavity) into which they pour 

 secretions that cause digestion outside of the cells. This 

 digestion in a cavity, as in the stomachs of all higher ani- 

 mals, is extra-cellular digestion, i.e., occurs outside of cells. 

 The digestive cells in Hydra, then, in part resemble the one- 

 celled animals and the digestive cells of sponge-animals ; and 

 in part the lining cells of the stomach of higher animals. 



Absorption of Digested Food. The cells (endoderm) which 

 line the digestive cavity of a hydra can either get food by 

 taking in particles and digesting them inside of cells, or by 

 absorbing foods digested in the cavity. The cells of the outer 

 layer (ectoderm) must get all their food by absorption or 

 osmosis from the endoderm cells. This osmosis of food 

 from cell to cell is as easily accomplished as water soaks 

 through several layers of paper. 



Use of food in cells of hydra is the same as in all other 

 animal cells. Some food is used to make new protoplasm for 

 repair and growth, and some serves as a source of energy. 



Oxygen is absorbed by all cells in contact with water. This 

 includes the majority of the cells, for even the digestive 

 cavity contains water taken in with the food. But probably 

 the larger part of the oxygen needed is absorbed from the 

 water by ectoderm cells. 



No Circulatory Organs. Although Hydra is multicellular, 

 it is able to live without circulating blood and lymph for the 

 reason that its simple cylindrical body allows digested food 

 and oxygen to reach all cells by osmosis, and all excretions 

 are eliminated by the same process. Compare this with 



