With a single exception to hundreds of observations upon six widely divergent genera of naked-eyed Medusæ, I find it to be uniformly true that removal of the extreme periphery of the animal causes instantaneous, complete, and permanent paralysis of the locomotor system. In the genus Sarsia, my observations point very decidedly to the conclusion that the principal locomotor centres are the marginal bodies, but that, nevertheless, every microscopical portion of the intertentacular spaces of the margin is likewise endowed with the property of originating locomotor impulses. In the covered-eyed division of the Medusæ, I find that the _principal_ seat of spontaneity is the margin, but that the latter is not, as in the naked-eyed Medusæ, the exclusive seat of spontaneity. Although in the vast majority of cases I have found that excision of the margin impairs or destroys the spontaneity of the animal for a time, I have also found that the paralysis so produced is very seldom of a permanent nature. After a variable period occasional contractions are usually given, or, in some cases, the contractions may be resumed with but little apparent detriment. Considerable differences, however, in these respects are manifested by different species, and also by different individuals of the same species. Hence, in comparing the covered-eyed group as a whole with the naked-eyed group as a whole, so far as my observations extend, I should say that the former resembles the latter in that its representatives usually have their main supply of locomotor centres situated in their margins, but that it differs from the latter in that its representatives usually have a greater or less supply of their locomotor centres scattered through the general contractile tissue of their swimming organs. But although the locomotor centres of a covered-eyed Medusa are thus, generally speaking, more diffused than are those of a naked-eyed Medusæ, _if we consider the organism as a whole_, the locomotor centres in the _margin_ of a covered-eyed Medusa are _less_ diffused than are those in the _margin_ of a naked-eyed Medusæ. In no case does the excision of the margin of a swimming organ produce any effect upon the movements of the manubrium. CHAPTER III. EXPERIMENTS IN STIMULATION. _Mechanical, Chemical, and Thermal Stimulation._