The method by which I determined the latent period in the case of this species was as follows. A basin containing the Medusa was filled to its brim with sea-water, and placed close beside a smoked cylinder, which, while it lay in a horizontal position, could be rotated at a known rate. The Aurelia[12] was placed with its concave aspect uppermost, and an inch or two below the surface of the water. The animal was held firmly in this position by means of a pair of compasses thrust through it and forced into a piece of wood, which was fastened to the bottom of the basin. The legs of the compasses were provided with india-rubber sliders, so that by placing these under the Medusa, the latter might be kept at any elevation in the water which might be desired. The manubrium and lithocysts were now removed, and also a segment of the umbrella. A light straw was then forced through the gelatinous substance of the umbrella in a radial direction, and close to the gap caused by the missing segment. The other, or free, end of this straw was firmly joined to a capillary glass rod, which was suitably bent to avoid contact with the rim of the basin, and also to write on the smoked cylinder. If the straw was not itself sufficient to support the weight of the capillary rod, a small cross-piece of cork might easily be tied to it, so as to add to the flotation power. A part of the excitable tissue was now raised above the surface of the water by means of a disk of cork placed beneath it, and on the part of the tissue thus raised there were placed a pair of platinum electrodes. These electrodes proceeded from an electro-magnetic apparatus, which was arranged in such a way that every time the current in it was opened or closed, it gave an induction shock and moved a lever at the same instant of time. This lever was therefore placed upon the cylinder immediately above the capillary glass-writer which proceeded from the Medusa, care being taken to place the two writers in the same line, parallel to the axis of the cylinder. Such being the arrangement, the cylinder was rotated, and thus two parallel lines were marked upon it by the two writers. If the current was now closed, an induction shock was thrown into the tissue at the same instant that the electro-magnet writer recorded the fact, by altering its position on the cylinder. Again, as soon as the paralyzed Medusa responded to the induction shock, the radii of the vacant segment were drawn apart, and in this way a curve was obtained by the other writer on the rotating cylinder. Now, by afterwards dropping a perpendicular line from the point at which the electro-magnet writer changed its position, to the parallel line made by the other writer, and then measuring the distance between the point of contact and the point on the last-mentioned line on which the curve began, the period of latent stimulation was determined. A glance at Figs. 3 and 4 (p. 55) will render this description clear to any one who is not already