ENDODERM 97 



oxygen and possibly of carbohydrates, and the plant benefited 

 by the rich supply of nitrogeneous material and carbon dioxide. 

 Such a partnership is known as symbiosis and is in contrast with 

 parasitism, in which one of the partners benefits at the expense 

 of the other. 



MOVEMENTS AND REACTIONS 



The movements of Hydra are carried out mainly by the 

 muscular processes of the cells, though the surface of the basal 

 disc can put forth pseudopodia, and it is possible that by means 

 of these the animal can slowly change its position. The muscular 

 processes of the ectoderm cells, when they contract, make the body 

 shorter and wider ; those of the endoderm make it narrower and 

 longer. The position of rest is one of moderate extension. The 

 green hydra does not remain passive in the absence of stimuli, 

 but, after standing for some time extended in readiness for prey, 

 it automatically contracts either the whole body or the tentacles 

 only, and then extends in a new direction. Thus it explores the 

 whole of its surroundings. From time to time it changes its 

 position. This is done by extending the body and bending it, 

 so that the tentacles touch some neighbouring object and adhere 

 to it by means of the nematocysts with sticky threads. The basal 

 disk is then either withdrawn altogether from the spot to which 

 it was fixed and put down in a new spot close to the tentacles, 

 or caused to ghde up to the tentacles. If the base is put down 

 beyond the tentacles the animal may be said to somersault 

 (Fig. 64). A hydra responds to every stimulus, except that of 

 food, by contracting. A stimulus applied to one side of the body 

 a number of times causes it presently to move away in some other 

 direction. Hydra avoids both too feeble and too strong a Hght. 



NUTRITION AND EXCRETION 



The food of Hydra consists of small animals, which are caught 

 by the tentacles, and carried by them to the mouth, which then 

 opens and swallows the prey. It is not pushed in by the tentacles. 

 Hydra is seldom able to kill water-fleas, though it can swallow 

 them once they are killed, and it can live indefinitely in water 

 in which there are no other animals large enough to be seen 



