HYDRA AND OBELI A. POLYPS AND MEDUSA 169 



porated by the protoplasm always contains complex organic 

 substances. 



The movements of Hydra are carried out mainly by the 

 muscular processes of the cells, though the 

 and Reactions, surface of the basal disc can put forth pseudo- 

 podia, 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. 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 sur- 

 roundings. 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 basal disc is then either withdrawn altogether from the spot 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 glide up to the tentacles. In 

 either case the animal moves in somewhat the same way as 

 a looper caterpillar. A Hydra responds to every stimulus, 

 except that of food, by contraction. If the stimulus be weak 

 it affects only the part of the body to which it is applied, 

 as a single tentacle will withdraw from a slight touch ; if it 

 be strong its effect spreads to the whole body. 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 light. 



The food of Hydra consists of water-fleas and other 

 small animals. These are caught by the 

 Excretio n n. and tentacles, and carried by them to the mouth, 

 which then opens and swallows the prey. The 

 animal will not feed unless it be hungry. If it be 

 well fed, creatures which swim against the tentacles are 

 allowed to escape, but, if food has been scarce, as soon as 

 the prey has become temporarily attached by the nemato- 

 cysts to one tentacle the others bend over towards it and 

 help to secure it and push it towards the mouth. If the 



