ORGANIC EVOLUTION 161 



the cell, with that of the higher animals, in which the food is 

 digested in a stomach through the action of fluids poured out 

 by the cells and afterwards absorbed. Individual cells of 

 the endoderm of the hydra digest minute diatoms, etc., after 

 the former method; the endoderm collectively digests 

 water-fleas and the larger kinds of prey after the latter 

 method. 



The ectoderm is a composite layer, especially in the upper 

 part of the body, consisting of larger cells that form the 

 surface layer, and taper down to their inner ends, and smaller 

 interstitial cells that fill the spaces between the bases of 

 larger cells, but do not reach to the outside. These inter- 

 stitial cells grow up to the surface and some of them develop 

 there into nematocytes by a remarkable differentiation of 

 the cytoplasm of their upper ends. It is probable that 

 after the discharge of the nematocysts, other of the inter- 

 stitial cells act as replacement cells, growing up and pro- 

 ducing new ones. 



There is a significant development of the base of the 

 ectoderm cells that can only be made out well if the tissues 

 be dissociated so that the cells may be viewed singly. In 

 such a preparation, many of the ectodermal cells are seen 

 to have long slender processes extending lengthwise of 

 the body beneath the ectodermal layer in physical contact 

 with many of its cells (fig. 101 e). It is believed that these 

 processes are especially contractile, and that they account 

 for the ability of the body as a whole to shorten so rapidly 

 when disturbed. These foreshadow the muscles of the 

 higher animals. Other cells of the form shown in figure 101 

 are also present among these processes and appear to be 

 primitive nerve cells, serving to communicate the stimuli 

 and to secure simultaneous contraction of all the cell pro- 

 cesses of the whole layer, securing concerted and coordinated 

 action. 



