1. Microstoma manipulates the nematocysts of Hydra either (a) as a means of eliminating indigestible parts of its food, or (b) a means of defending itself. 2. The fact that the nematocysts are distributed uniformly over the surface of Microstoma within 12 hours after a Hydra has been ingested, suggests that this is not a process of elimination, (a), but is done in order that the nematocysts may be used, (b). 3. The fact that the nematocysts are retained indefinitely suggests that they have been taken up and retained for use. 4. The fact that this method of handling the nematocysts demands considerable work on the part of the endodermal cells and, in each case of a discharged nematocyst, the death of a cnidophage or mesodermal cell further suggests that it is all done with reference to use. 5. Microstoma incurs danger in attacking Hydra; for it is frequently stung and sometimes killed by the polyp. This too suggests that the Microstoma attacks Hydra for some peculiar end. 6. The Microstoma is able to draw nematocysts with which Hydra has stung it from its body, without suffering a rupture as does Stenostoma. 7. Microstoma with a few nematocysts attacks Hydra within a little time. 8. Microstoma having many nematocysts does not readily attack Hydra. The contrast between 7 and 8 suggests that Microstoma attacks Hydra not for food; but for its nematocysts. 9. The alimentary canal of a recently fed Microstoma, that has few nematocysts, may egest the "flesh" of Hydra and retain its nematocysts. 10. The alimentary canal of a Microstoma that has many nematocysts and is hungry, may egest the nematocysts of Hydra and retain the "flesh" of the polyp. The contrast between 9 and 10 suggests that Microstoma seeks primarily the nematocysts of Hydra. 11. The discharge of the nematocysts is a double process, involving (1) an oscillation to and fro of the nematocyst within its cnidophage, and (2) the actual discharge. The first phase may be carried on without being followed by the second phase. Neither of these has ever been seen except when some active body, like the experimenter's needle or an animal has brushed along the surface of Microstoma. This, too, suggests that the nematocysts are for use. 12. Finally Microstoma actually stings and paralyses other animals with the nematocysts it has appropriated from Hydras that it has eaten.