282 
ME. a. J. ROMANES ON THE LOCOMOTOR SYSTEM OE MEDTTSJE. 
cases). It is needless to observe how strongly this fact points to the ganglionic nature 
of the locomotor centres in the marginal tissues of Medusae. 
My observations were principally made upon the genus Sarsia ; and the method I 
employed was to pass the platinum electrodes into the concavity of the mutilated 
swimming-bell while the latter was in the water, and then gently to raise the former 
until the contact between them and the tissue was seen to be sufficiently intimate. 
Similarly of course with the severed margin. I may here remark, for the benefit of 
those who may repeat this observation, that when the constant current is being applied 
to the mutilated bell the latter often contracts in a somewhat rhythmical manner. 
This is perhaps due to the hydrogen bubbles acting as stimulants to contraction, 
and in any case is certainly not to be regarded as spontaneous action. I may also 
state that minute crustaceans, by striking the mutilated bell, sometimes supply a 
stimulus to contraction. It is therefore desirable to conduct these experiments in 
filtered sea-water; and the same precaution of course should be taken when conducting 
the fundamental observation upon such swimming-organs as prove themselves highly 
sensitive to stimulation after removal of their locomotor centres. 
(B) Excitable tracts . — The extreme sensitiveness of all the tissues of all the Medusae 
to electrical stimulation affords us the means of ascertaining whether there is 
any localization of definite excitable tracts in these animals. As Sarsice are the most 
active of the Medusae, and likewise the most delicately sensitive to electricity*, I have 
in this, as in other cases, taken the genus as the type of the group, and made it the 
subject of a more careful investigation than any of the other genera. In this prelimi- 
nary paper, therefore, I shall confine myself, under this heading, to detailing my 
observations upon the genus Sarsia alone. The method I adopted was to slit open one 
side of the swimming-bell from base to apex, and then to lay it flat upon a glass slide 
with its inner surface uppermost. Having either cut off or turned back the polypite, I 
then placed the entire animal under the microscope on a slightly grooved object-glass, 
where I could keep it alive for a considerable time by moistening it at intervals with 
drops of sea-water. The stimulus I employed was always the induction-shock supplied 
by a single Daniell’s cell and Du Bois-Reymond’s coil. The electrodes were fine 
needles passed through a small piece of india-rubber. The latter was firmly fixed to 
the stage forceps, which in turn was firmly fixed to a mechanical stage. In this way 
the object and the electrodes could be moved in any direction without altering their 
relative positions — a provision which made all the difference between the following 
observations being possible or impossible ; for without this provision it would not have 
* At whatever point the bell of Sarsia first responds to the induction-shock, it will generally be found that 
drawing out the secondary coil a quarter, or even an eighth, and very often only a sixteenth of an inch will 
make all the difference between a vigorous response and no response. Moreover this delicacy of appreciation, 
besides being thus very great, is also very constant : the excitability of the tissue is seldom found to vary in 
the least degree for a tolerably long time, although, of course, under the unnatural conditions in which the 
animal is placed, the excitability of its tissues begins at last gradually to diminish. 
