THE REACTION OF HYDRA TO INANITION

Hydra, as a diploblastic animal, can have no circulatory medium. Perhaps for a similar reason it can have no storage tissue such as fat. Green Hydras seem able to fall back upon the surplus foods of its zoöchlorellæ, for, contrary to some observers, it has been our experience that Hydra viridis is much less influenced by inanition than other species. In all species, if the inanition be prolonged and the individuals be kept free from parasites and concentrated toxins, each specimen not getting food, will begin to feed upon its tentacles. First the ends of the tentacles are bitten off. If now food is yet with-held from the Hydra, it will feed upon its tentacles until but the merest stumps are left. The ingested tentacles will be digested. Even the nematocysts (both types) will be taken into food-vacuoles within the epithelio-muscular cells and be completely digested and absorbed. After the Hydra has thus fed upon and appropriated its tentacles—except for mere stumps that stand about the peristome—the bases of the amputated tentacles will regenerate and the Hydra, now reduced in size, will possess a new group of tentacles. In feeding upon the tentacles that have been ingested, the endoderm of the stumps of tentacles appropriate relatively more of the ingested tentacle-material than is appropriated by the endoderm of the body proper. This appears to be correlated with the fact that regeneration is to take place in the tentacular stumps.