HYDRA. 135 



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 tentacle, or a 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 x'Vbbe 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 intern- 

 ally, 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 devel- 

 oped. 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 continuous with 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 

 female organs may occur on the same animal, either at one 

 time or at different times, but often they occur on different 

 individuals. The buds have the same structure as the parent 

 body, but in origin they appear to be wholly ectodermic. 



Minute .Structure. — The outer layer or ectoderm incliulcs the follow- 

 ing different kinds of cells : — 



(l) Large covering or epithelial cells, within or between some of 



