86 The Insides and the Workjngs of Living Things 



at their bases, which spread between the two layers and are 

 highly contractile: they may be considered primitive muscles. 

 Other cells of the outer layer and a small number of the inner 



layer project slightly beyond 

 the surface and are especially 

 sensitive to disturbances : they 

 may be considered as primi- 

 tive nerve or sensory cells. 



We may note that in the 

 hydra as in the ameba each 

 cell assimilates food and 

 grows, each cell is more or 

 less irritable, more or less 

 contractile. Moreover, each 

 cell of the hydra is capable 

 of dividing into two cells, as 

 is the ameba; and the growth 

 of the body results from the 

 increase in the number of 

 cells as well as from the en- 

 largement of the individual 

 cells. On the other hand, 

 hydra shows what has been 

 called a division of labor. 

 Some cells carry on certain 

 functions almost exclusively, 

 or to a greater extent than 

 some of the other cells. We 

 may say there is a tendency for cells to specialize; and at the 

 same time there is a certain interdependence among the 

 different parts. 



Fig. 20. The Hydra 



This animal consists of two layers of 

 cells. The base of the sac is usually at- 

 tached to some support, such as a stone or 

 a bit of some plant. At the open end are 

 several hollow extensions or tentacles, 

 which wave about in all directions. The 

 " mouth " receives food particles, which 

 may be digested in the cavity. The food 

 prepared in the central cavity is made 

 available to all the cells. 



From General to Special 



A comparison of all kinds of animals yields this general 

 fact: we may arrange the various types to indicate a progres- 

 sive increase in specialization, as from the one-celled animals 



