THE JELLYFISH AND THEIR ALLIES 



3465 



been met with in brackish water on the coasts of Europe and of North America. 

 After that it appeared from time to time in the lower courses of rivers, such as the 

 Thames, the Elbe, etc. , and now it has found its way far inland both in the Old 

 and New Worlds. It occurs in the Saale, near Halle, and is specially plentiful in 

 the slightly brackish lake of Eisleben. In Hamburg it has in some places invaded 

 the water pipes supplying the city, developing in them to such a degree as actually 

 to stop them up. This history of the migration of Cordilophora is instructive in 

 helping us to understand the rise of at least a part of the fresh-water fauna. In 

 this case, within our own experience, an animal inhabiting brackish water has in a 

 few years become so adapted for living in fresh water as to be considered altogether 

 \ fresh- water form, without the least apparent change in its organization. 

 Whether a change in organization would not 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 Hydradinia 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 ten- 

 tacles like fine threads. Small creatures, com- 

 ing in contact with these tentacles, remain 

 attached to them, caught and held by the 

 stinging threads, whereupon the tentacle con- 

 tracts, bringing the prey to the mouth, which 

 is capable of great extension. Besides the 

 large stinging cells which shoot out long poi- 

 sonous threads, paralyzing 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 ejec- 

 tion of the long threads. Jickeli, who closely 

 investigated this matter, came to the con- 

 clusion that the small cells were modified for 

 an entirely different function. However small 

 the little crustaceans paralyzed 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 

 neutralize 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 



HYDRA MONSTER, ARTIFICIALLY PRO- 

 DUCED. (5 times natural size.) 



