150 BIOLOGY: GENERAL AND MEDICAL 



advantageous. Among animals, where movement is 

 of prime importance, it becomes indispensable. The 

 more highly specialized any cell group becomes, and the 

 more completely isolated it becomes in consequence, 

 the more imperative becomes the necessity for communi- 

 cation, control, and coordination. 



In loosely organized cellular combinations, such as 

 Epistylus and Microgromia, little advantage is to be 

 gained for one cell by impulses derived from others, 

 though the general irritability and conductivity of the 

 protoplasm may enable impulses to pass from cell to 

 cell. When one cell in such a simple colony is disturbed, 

 a defensive reaction is manifested by its fellows, and in 

 the case of Carchesium may result in escape from danger 

 through contraction of the stalk. In such cases, as well 

 as in the sponges and in hydra, the threatened danger 

 is, however, usually of little importance to other cells than 

 those immediately menaced by it. If those attacked 

 should be destroyed a portion of the sponge torn away 

 or a tentacle of the hydra bitten off the whole organism 

 is scarcely affected and the damage is soon repaired. 

 The same conditions obtain among plants, so that the 

 entire development of the plant kingdom has progressed 

 without any regulating or communicating i.e., nervous 

 mechanism. The importance of movement among 

 animals has been dwelt upon, and we find means of 

 controlling it making their appearance very early. 

 Thus, of the ectodermal cells of hydra we find that 

 though it is probably true that all of the cells are sensi- 

 tive i.e., irritable certain of them, called neuro- 

 muscular cells, exceed their fellows in sensitivity and con- 

 tractility, and probably act as guides or indicators by 

 which movements, especially of the tentacles, are directed. 



Among the higher ccelenterates these ectodermal cells 

 appear to transmit the impulses they receive to certain 

 specialized "nerve cells" subjacent to them, and these, 

 in turn, excite muscle cells through the mediation of 

 certain fibres extending from one to the other. 



