In another hydroid, _Margelis carolinensis_, studied by Loeb, the effect of contact is more easily demonstrated. If a branch of margelis is put into a dish of water and is kept from all motion, the parts that come in contact with the dish produce roots that attach themselves. Even the apical end of the stem may grow out as a root, as shown in Fig. 15, _A_. Those parts of the branch that are not in contact with any solid object give rise to new hydranths. Another hydroid, _Pennaria tiarella_, also shows, according to Loeb, the same response to contact. In this connection it is interesting to find that a growing hydranth of pennaria, if brought in contact with a solid body, turns away from the region of contact and bends at right angles to the body which it touches. We find, once more, that a factor having an influence on the growth of the animal has also a similar influence on the regeneration. Loeb has found that if pieces of the hydroid _Campanularia_ are cut off and placed in a dish filled with sea water, all the hydranths that touch the bottom of the dish are absorbed and transformed into the substance of the stem. The cœnosarc may creep out of the stem wherever it comes in contact with the glass, and produce stolons that give rise to new polyps on their upper surfaces. Loeb shows that growth takes place at the end of the stolon that pushes out of the perisarc, and this growing region draws the rest of the cœnosarc after it. If a new hydranth appears along the old piece, the cœnosarc is drawn towards the hydranth. _EFFECT OF CHEMICAL CHANGES IN THE ENVIRONMENT_