Page:Popular Science Monthly Volume 14.djvu/331

Rh flash was of slightly more than one second's duration, no response was given. That is to say, the minimal duration of a flash required to produce a responsive spasm was just the same as the time during which a continuous flood of light required to operate in order to produce a similar spasm. From this, therefore, I conclude that the enormously long period of latent excitation in the case of luminous stimuli is not, properly speaking, a period of latent excitation at all; but that it represents the time during which a certain summation of stimulating influence is taking place in the ganglia, which requires somewhat more than a second to accumulate, and which then causes the ganglia to originate an abnormally powerful discharge. So that in the action of light upon the ganglionic matter of this medusid we have some analogy to its action on certain chemical compounds in this respect—that, just as in the case of those compounds which light is able to split up, a more or less lengthened exposure to its influence is necessary in order to admit of the summating influence of its vibrations on the molecules; so in the case of this ganglionic material, the decomposition which is effected in it by light, and which terminates in an explosion of nervous energy, can only be effected by a prolonged exposure of the unstable material to the summating influence of the luminous vibrations. Probably, therefore, we have here the most rudimentary type of a visual organ that is possible; for it is evident that, if the ganglionic matter were a very little more stable than it is, it would either 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!

Before leaving the case of luminous stimulation, I may observe that some of the Medusæ appear to be very fond of light. For, on placing a number of Sarsiæ in a large bell-jar in a dark room, and then throwing a beam of light through a part of the water in the bell-jar, the Medusæ all crowded into the path of the beam, and dashed themselves against the glass nearest to the light, very much as moths might do under the influence of similar stimulation. On moving the lamp round the jar, a cluster of Medusæ always followed it. This latter experiment is important, because it proves that the marginal ganglia are so far coordinated in their action that they can steer the animal in any particular direction.

Staurophora laciniata is a large species of naked-eyed medusa, which responds to stimulation in two very different ways, according as the stimulation is applied to the nervo-muscular sheet, or to the marginal ganglia. For, if the stimulation is applied to the nervo-muscular sheet, the response is an ordinary locomotor contraction; whereas, if the stimulation is applied to the marginal ganglia, the response is a