Marshall found that if the head of a hydra is partially split in two, each half-head completes itself (as Trembley had already shown). The body then begins slowly to separate into two parts, beginning at the angle between the two heads, until finally the two parts completely separate. King (1900) has repeated the experiment in a large number of cases with the same result. It seemed that the division might be brought about by the weight of the halves causing the gradual separation of the body, but King has shown that this is not the case, for, when a double form remained hanging with its head down, it still divided into two parts (Fig. 47, _A_). In this case, the weight of the two heads would cause the parts to come together rather than to separate, if gravity had any influence of the sort suggested. Marshall and King have also shown that if the posterior end of a hydra is split in two, the two parts do not continue to separate, but one of the two, if the pieces have been split some distance forward, may become constricted from the other, and, producing new tentacles at its apical end, become a new individual. I have carried out a series of experiments on planarians of a somewhat similar nature. If the posterior end is split in two, the separation extending into the anterior part of the worm (Fig. 44, _C_), each half completes itself, but the halves do not separate unless they happen to tear themselves apart. If one of the pieces is cut off, not too near the region of union with the other half, a new posterior end, replacing that cut off, regenerates. If, however, the piece is cut off quite near the region or union of the halves, the piece that is left may be absorbed. The absorption of misplaced parts in the lower animals cannot be explained, I think, by any lack of nutrition, especially in the case of the tentacles of hydra. The result may be due either to the displaced part not receiving exactly those substances, perhaps food substances, that it gets in its normal position, or it may be due to some formative influence. At present we are not in a position to decide between these alternatives, and, while the former view seems more tangible, and the latter quite obscure, the latter may nevertheless be found to contain the true explanation. If the view that I have adopted in regard to the organization--namely, that it can be thought of as acting through a system of tensions peculiar to each kind of protoplasm--is correct, it may be possible to account for the absorption of misplaced parts by some such principle as this. _INCOMPLETE REGENERATION_ A somewhat unusual process of regeneration takes place when the jelly-fish, _Gonionemus vertens_, is cut into pieces. As first shown by Hargitt, the cut-edges come together and fuse, and the pieces