TYPES OF CCELENTERA HYDRA. 



139 



except those concerned in reproduction. The body is 

 usually fixed by its base to some aquatic plant, often to 

 the lower surface of a duckweed. It may measure \-\ inch 

 in length, but it is as thin as a needle, and contracts into a 

 minute knob. 



The animal sways its body and tentacles in the water, 

 and it can also loosen its base, lift itself by its tentacles, 

 stand on its head, or creep by looping movements. Ac- 

 cording to some observers, its movements may be helped 

 by fine pointed pseudopodia protruded from the ectoderm 

 cells of the tentacles and base. Usually, however, the 

 Hydra prefers a quiet life. It feeds on small organisms, 

 which are paralysed or killed by stinging cells on the 

 tentacles, and are swept into the 

 tubular cavity of the body by the 

 action of flagella on the internal cells. 

 Sometimes animals as large as water- 

 fleas (e.g. Daphnia) are caught, and in 

 part digested. Infusorians (Euplotes, 

 etc.) are often seen wandering to and 

 fro on the surface of the Hydra, but 

 these wonted visitors do not seem to 

 provoke the stinging cells to action. 



So simple is Hydra, that a cut-off 

 fragment, containing samples of the 



9 ,' , ,1 11 /] FIG. 64. Hydra hang- 



vanous kinds or cells in the body, and in? from wa ter-weed. 

 not too minute, may grow into an -After Greene, 

 entire animal. Thus the Hydra may ov .. ovary ; *.,testes. 

 be multiplied by being cut in pieces. 

 The two conditions of a fragment regenerating a whole are 

 -(i) that the fragment be not too small, and (2) that it be a 

 fair sample of the whole animal. Thus the tip of a 

 tentacle will not grow into a new Hydra. If the animal be 

 turned inside out (a delicate operation), the status quo is 

 soon restored. The Abbe Trembley, who first made this 

 experiment, thought that the out-turned inner layer or 

 endoderm assumed the characters of the outer layer or 

 ectoderm, and that the inturned ecto.derm assumed the 

 characters of endoderm. But this is not the case. Either 

 the animal rapidly rights itself by turning outside in, or, 

 if this be prevented, the inturned ectoderm disappears 



