JELLY-FISH, ETC. 



491 



gradually take place in the course of many years is, of course, another question, 

 which for the present is unanswerable. 



In the Hydra we have a hydropolyp much better known and much more 

 specially adapted to its habitat than the Cordilophora. These hydras, which are 

 from one-eighth to one-third of an inch in length, form simple stocks of one or two 

 branches, and as often as not are found single. They almost exactly resemble in 

 form those polyps of the Hydr actinia which are provided with a circle of tentacles. 

 The water of stagnant pools or ponds in which water-plants are abundant will 

 almost always yield one of the three species of the fresh-water hydra, if the water- 

 plants be left undisturbed in a vessel. The little creatures often leave the weed and 

 attach themselves to the sides of the vessel, where they can be examined with a lens. 

 When undisturbed, the polyps begin to extend 

 and spread out their six or eight tentacles 

 like fine threads. Small creatures, coming in 

 contact with these tentacles, remain attached 

 to them, caught and held by the stinging- 

 threads, whereupon the tentacle contracts, 

 bringing the prey to the mouth, which is 

 capable of great extension. Besides the large 

 stinging-cells which shoot out long poisonous 

 threads, paralysing and holding fast the small 

 creatures that happen to come too near, the 

 hydra also possesses a smaller kind of cells 

 with smooth threads which are not ejected by 

 the stimulus that leads to the ejection of the 

 long threads. Jickeli, who closely investigated 

 this matter, came to the conclusion that the 

 small cells were modified for an entirely dif- 

 ferent function. However small the little 

 crustaceans paralysed by the hydra may appear to us, relatively to the hydra 

 they are enormous, and, on being stung, would sink heavily to the bottom. 

 Jickeli's observations led him to think that the smaller capsules act as buoys to 

 neutralise the action of gravitation. Indeed, when we remember how far removed 

 tentacles are from being hands, we can understand how much more easily a victim 

 could be got into the mouth if it floated helplessly near, than if it tended at every 

 moment to sink like a stone. The hydra usually multiplies by means of buds 

 which grow out of the body. The offspring often remains attached to the mother 

 until it, in its turn, has given rise to one or two buds. Single eggs, however, develop 

 from time to time in the body- wall beneath capsule or wart-like prominences. 



The astonishment of the naturalist Trembley, when he discovered that a 

 hydra cut in pieces was not destroyed, but that the pieces were capable of 

 developing into new individuals, was great. He thought that if the hydras were 

 plants, pieces cut from them would, like young shoots, be capable of further 

 growth. But he had, meantime, come to the conclusion that they were animals, 

 and according to the ideas of the time it was an unheard-of thing that new 

 individuals could grow from cut-off pieces. And thus commenced his experiments 



HYDRA MONSTER, ARTIFICIALLY PRODUCED 



(5 times nat. size). 



