144 PHYLUM CCELENTERA. 



etc.) are often seen wandering to and fro on the surface 

 of the Hydra^ but these wonted visitors do not provoke the 

 stinging cells to action. 



So simple is Hydra, that a cut-off fragment may grow 

 into an entire animal. Thus the Hydra may 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 various kinds 

 of cells in the body. Thus neither a little corner off the 

 base nor the tip of a tentacle will 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 

 endoderm assumed the characters of the 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. Sometimes a Hydra divides across 

 the middle, and each half grows into a complete polyp in 

 a few days. Besides these asexual modes 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 much 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 trans- 

 parent, 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 farther 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 



