HYDRA 67 



germ layers that all the tissues and organs of the animal body are 

 finally formed. 



4. Adaptation 



The simple nervous system which we have indicated in Hydra 

 represents a great forward step in animal adaptation. Through 

 this irritable and conductile tissue Hydra is able to receive ex- 

 ternal stimuli and to respond, or adapt, by the proper coordinated 

 actions. It is, of course, apparent that all the cells in an animal 

 must work together for the good of the organism as a whole or it 

 cannot survive. It is the nerve tissue which governs this, unifying 

 the various parts of an organism so that this indispensable con- 

 dition is attained. Take, for example, locomotion in Hydra. 

 This is brought about by means of a coordinated movement of 

 the muscular elements in various parts of the body under the 

 direction of the nerve cells. Locomotion takes place in several 

 different ways : first, the Hydra may move by a slow, creeping 

 movement in which the epithelio-muscular cells of the basal disc 

 alone are involved ; second, the Hydra may stand on its head, 

 so to speak, and move along by a coordinated movement of its 

 tentacles, or, finally, it may move like a ' measuring worm ' by 

 using both the basal disc and the tentacles. Whatever the type 

 of locomotion, the actions of all the cells concerned are under the 

 control of the nerve tissue, and the same is true for all the ac- 

 tivities of the animal. 



TEXTBOOK REFERENCES 

 Woodruff, pp. 61-66; 98-100; 229-237. 



Curtis and Guthrie, pp. 247-274 ; 317-320. 

 Guyer, pp. 197-204 ; 345-348 ; 667. 

 Hegner, pp. 102-112. 

 Newman, pp. 112; 160-176. 

 Shull, pp. 75-95; 131. 



GENERAL REFERENCES 



Hickson. " Coelenterata and Ctenophora," in the Cambridge Natural History 



(Macmillan). 

 Hickson. An Introduction to the Study of Recent Corals (Longmans). 

 Morgan. Field Book of Ponds and Streams (Putnams). 

 Parker and Haswell. Textbook of Zoology (Macmillan). 



