684 
MR. Gr. J. ROMANES ON THE LOCOMOTOR SYSTEM OF MEDUSAE. 
of light of measured duration. I found that unless the flash of light was of slightly 
more than one second in its 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 response to luminous stimuli was not, properly speaking, a period 
of latent excitation at all, but that it represented the time during which a certain 
summation of stimulating influence was taking place in the ganglia, which required 
somewhat more than a second to accumulate, and which then caused the ganglia to 
originate an abnormally powerful discharge. 
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. 
§ 5. Electrical stimulation. (A) Latent periods and characters of the subsequent 
contractions, (a) Method. — Under this heading I propose to treat at the same time of 
the period of latent stimulation and the manner in which this, as well as the responsive 
contractions, are modified by certain conditions. As Aurelia aurita is the only species 
on which I have experimented in this connexion, my remarks under this heading 
will be confined to it alone. The method by which I determined the latent period in 
the case of this species was as follows. A basin containing the Medusid was 
filled to its brim with sea- water, and placed close beside a cylinder which, while it lay 
in a horizontal position, could be rotated at a known rate. The Aurelia * 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 sunk at 
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 polypite and lithocysts were 
now removed, and also a segment of the disk. 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 floatation-power. A part of the excitable tissue was now raised 
* It may here be stated that, in all the experiments on stimulation subsequently to he detailed, there is no 
difference to be observed between the behaviour of an entire swimming-organ deprived of its ganglia and that 
of a portion of any size which may be separated from it. 
