THE HYDRA. 31 



motion, which may be seen in thin transverse sections 

 cut from a living specimen. Their combined motions 

 probably have much to do with comminuting the food. 

 Their secretions probably act on it chemically to digest 

 it; but when it is digested, it must pass out to all parts 

 of the body by simple osmosis from cell to cell, and by 

 the slight propulsion given by the moving of the parts 

 of the body upon one another, for there are no circulating 

 vessels. There being but one external opening to the 

 digestive tract, the indigestible portion of the food must 

 be cast out, as it is taken in, through the mouth. The 

 absorption of free oxygen and of mineral foods in solu- 

 tion in the water, takes place directly through the body 

 wall. The assimilation of these various foods from vari- 

 ous sources, the building of them into the body structure, 

 and the removal of the waste products of activity, 

 these things, in hydra, as in all animals, each cell does 

 for itself. 



II. Reproduction. That .the hydra is hermaphrodite 

 has already been seen, and also that it reproduces 

 greatly by budding. Did these buds remain perma- 

 nently attached, as in some of the hydra's marine rela- 

 tives, instead of falling away when fully grown, we 

 should have it developing a peculiar treelike form. 

 Hydra does not divide of itself into two equal parts, but, 

 when so divided artificially, the parts may each become 

 a new hydra with all lost parts reproduced. Pieces much 

 smaller than half the animal may grow into perfect hy- 

 dras. It seems to be only necessary that all the essential 

 structures be represented, and probably is most essential 

 that the ectoderm (the reproductive layer) and the erido- 

 derm (the digestive layer) be present and in their proper 

 relations together. 



III. Voluntary Motion. The hydra is another of the 

 animals that "move without muscles." Yet its move- 



