very little more stable than it is, it would altogether fail to be thrown down by the luminous vibrations, or would occupy so long a time in the process that the visual sense would be of no use to its possessor. How great is the contrast between the excitability of such a sense-organ and that of a fully evolved eye, which is able to effect the needful molecular changes in response to a flash as instantaneous as that of lightning. [10] The period of latent stimulation merely means the time after the occurrence of an excitation during which a series of physiological processes are taking place, which terminate in a contraction; so that, whether the excitation is of a strong or of a weak intensity, the period of latent stimulation is not much affected. The above question, therefore, was simply this--Does the prolonged delay on the part of these ganglia, in responding to light, represent the time during which the series of physiological processes are taking place in response to an adequate stimulus, or does it represent the time during which light requires to act before it becomes an adequate stimulus? With regard to luminous stimulation, it is only necessary further to observe that responses were given equally well to direct sunlight, diffused daylight, and to light reflected from a mirror inclined at the polarizing angle. It must also be stated that responses are given to any of the luminous rays of the spectrum when these are employed separately; but that neither the non-luminous rays beyond the red, nor those beyond the violet, appear to exert the smallest degree of stimulating influence. _Electrical Stimulation._ All the excitable parts of all the Medusæ which I have examined are highly sensitive to electrical stimulation, both of the constant and of the induced current. Exploration with needle-point terminals and induction shocks of graduated strength showed that certain parts or tracts of the nectocalyx are more sensitive than others. The most sensitive parts are those which correspond with the distribution of the main nerve-trunks, _i.e._ round the margin of the nectocalyx and along the course of the radial tubes. The external or convex surface of a nectocalyx or umbrella is totally insensitive to stimulation, and the same statement applies to the whole thickness of the gelatinous substance to which the neuro-muscular sheet is attached.