86 ESSENTIALS OF ZOOLOGY 



system, but nevertheless currents carry the digested food from 

 the endoderm through the mesogloea to the ectoderm. It 

 has been shown by experiment that a starving Hydra may 

 lose mouth and tentacles, a remarkable result of the cessation 

 of, or the reversion of, the osmotic current an indication also 

 of a return to a simpler condition. In each cell the food 

 obtained in a fluid state is reassembled by further enzymotic 

 action to suit the condition of the cell, and in each it is liable 

 to oxidation in the production of work. Both the ectoderm 

 and the endoderm are highly contractile. 



Psychologically the condition of Hydra is a lowly one. 

 The cells are undoubtedly related by contact in a nervous 

 manner, the epithelial cells by protoplasmic threads probably, 

 and besides a generalised nervous system is outlined. The 

 cells, however, in their processes and in their reactions are to 

 a large extent independent. Local contraction is also mani- 

 fested in the bending of the body and of a tentacle. 



Reproduction. Owing to the small degree of specialisa- 

 tion and the universal presence of the two layers in all parts 

 of the column and tentacles, Hydra regenerates readily from 

 sections. Cut in two, the lower half of the body reproduces 

 the head and the upper half the base, and in all cases the 

 original orientation is preserved. This was described in 1744 

 by Trembley, 1 but his statement that the Hydra could be 

 turned inside out, and that the ectoderm became the new 

 endoderm and the endoderm the ectoderm, is not believed, 

 and experiments of the same kind have not been followed 

 by this result. 



Fission may take place either horizontally or vertically, 

 but this method of vegetative reproduction has not been often 

 observed. 



During the summer, when food is plentiful, growth takes 

 place rapidly, and new individuals are produced by hollow 

 buds emerging from the side of the column. A bulging-out 

 takes place of the two layers, and the bud grows as a new 

 column and forms a mouth and tentacles at the free end. A 

 secondary bud may appear before the new hydra is freed. 

 The separation takes place by the base of the daughter hydra 



1 loc. dt. 



