TYPES OF CCELENTERA HYDRA. 131 



cells of the tentacles, etc. 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. Daphnid) 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 sting- 

 ing cells to action. 



So simple is Hydra, that a cut-off fragment, containing 

 samples of the various kinds of cells in the body, and not 

 too minute, may grow into an entire animal. Thus the 

 Hydra may be multiplied by being cut in pieces. If the 

 animal be turned inside out (a delicate operation), the status 

 quo is soon restored. The Abbd 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 ectoderm 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 

 internally, and, by growing over the out-turned endoderm, 

 from the lips downwards, restores the normal state. 



In favourable nutritive conditions, the Hydra forms buds, 

 and on these a second generation of buds may be developed. 

 A check to nutrition or some other influence causes the 

 buds to be set adrift. Besides this asexual mode of 

 multiplication, the usual sexual reproduction occurs. 



General structure. — The tubular body consists of two 

 layers of cells, i.e. the animal is diploblastic. The cavity 

 is the gut, and it is continued into the hollow tentacles. 

 These, when fully extended, may be longer than the body. 

 The mouth is slightly raised on a disc or hypostome. Of 

 the two layers of cells, the outer or ectoderm is transparent, 

 the inner or endoderm usually contains abundant pigment. 

 On the tentacles especially, even with low power, one can 

 see numerous clumps of clear stinging cells. The male 

 organs appear as ectodermic protuberances a short distance 

 below the bases of the tentacles ; the ovary, with a single 

 ovum, is a larger bulging further down. Both male and 



