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 Cordilojjhora. 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 Hydractinia 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). 
