ME. Gr. J. ROMANES ON THE LOCOMOTOR SYSTEM OF MEDUSAS. 
685 
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 made 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 para- 
lyzed 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 at which the curve began, the period of latent 
stimulation was determined. A glance at figs. 2 & 3 will render this description clear 
to any one who is not already acquainted with the method, when it is stated that the 
upper line is a record of the movements of the electro-magnet writer, and the lower 
line that of the movements of the other writer. It will be observed that the point 
a in the upper line marks the point at which the induction-shock was thrown in ; 
so that by first producing the perpendicular till it meets the lower line at b, and then 
measuring the distance between the point b and the point c, at which the curve in 
the lower line first begins, the latent period (be) is determined — the time occupied by 
the rotation of the cylinder from b to c being known. 
(b) Summation of Stimuli . — In this way I have been able to ascertain the period of 
latent stimulation in Aurelia aurita with accuracy. It must be stated at the outset, 
however, that this period is subject to great variations under certain varying con- 
ditions ; so that we can only arrive at a just estimation of it by understanding the 
nature of the modifying causes. To take the simplest cause first, suppose that the 
paralyzed Aurelia has been left quiet for several minutes in sea-water at 45°, and that 
it is then stimulated by means of a single induction-shock. The responsive contraction 
will be comparatively feeble with a very long period of latency, viz. f of a second. If 
another shock of the same intensity be thrown in as soon as the tissue has relaxed, a 
somewhat stronger contraction with a somewhat shorter latent period will be given. 
If the process is again repeated, the response will be still more powerful, with a still 
shorter period of latency ; and so on for perhaps eight or ten stages, when the maximum 
force of contraction of which the tissue is capable will have been attained, while the 
period of latency will have been reduced to its minimum. This period is f of a second, 
or, in some cases, slightly less. 
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