ME. G. J. EOMANES ON THE LOCOMOTOE SYSTEM OF MEDUSAE. 
693 
action in Sarsia is of a very marked and unmistakable character. I may begin Fig. 5. 
by stating that when any part of the internal surface of the bell is irritated, the - ^,1 
polypite responds ; but as there is no evidence of ganglia occurring in the poly- miu V 
pite, this cannot properly be regarded as a case of reflex action. But, now, the 
converse of the above statement is likewise true, viz. that when any part of the I 
polypite is irritated, the bell responds; and it is in this that the unequivocal nft.’size. 
evidence of reflex action consists. Of course the objection at once arises, if response by 
the polypite to irritation of the bell is explained by the simple supposition of there being 
a continuous or common excitability pervading these organs, why may not the converse 
case be explained by the same supposition l But this objection is met by a fact that makes 
an essential difference between the two cases, viz. that while the sympathy of the polypite 
with the bell is not in the least impaired by removing the marginal ganglia of the latter, 
the sympathy of the bell with the polypite is by this operation entirely destroyed. 
We have thus very excellent demonstration of the occurrence of reflex action in the 
Medusse ; and as this is such an admirable instance of it, I shall describe it a little more 
in detail. Suppose the experimenter to have chosen a vigorous specimen of 8arsia with 
its polypite well retracted, and suppose him to wait for the occurrence of one of the 
natural pauses in the swimming-motions ; if he then suddenly seizes the end of the 
polypite with a pair of forceps, taking care not to touch any part of the bell, the 
polypite is instantly retracted even further than it was before, and immediately 
afterwards the tentacles are drawn up while the bell begins to make violent efforts 
to escape. But if the experimenter chooses a specimen having its polypite somewhat 
relaxed, and, after seizing the free end of this organ, follows it up with the forceps 
during its retreat, he may observe, if he has been dexterous enough not to allow the 
slightest pull to occur at the point of suspension of the polypite, that the bell does 
not respond to the stimulus however severe this may have been. By repeating this 
experiment a number of times, it becomes evident that the reflex action occurs, not 
between the marginal ganglia and every part of the polypite, but between the marginal 
ganglia and the point of the bell from which the polypite is suspended. In other words, 
the marginal ganglia do not respond to any amount of disturbance that may be set up 
in the polypite itself, provided the contraction of this organ does not cause it to pull 
upon the point of its attachment with the bell. Now the high degree of sensitiveness 
shown by the ganglia to the smallest degree of such traction is quite as remarkable as 
is their lack of sensitiveness to disturbances going on in the polypite*. Consequently 
* When a specimen of Sarsia is taken out of the water, laid on an object-glass, slit open along one side, and 
spread out flat with its inner surface uppermost, and then has its polypite turned hack as represented in the figure 
on p. 697, it may he observed that the hell often responds to irritations of the polypite, even though care he taken 
to release the latter from the forceps before it is allowed to pull on its point of suspension. This may possibly 
he due to the fact of the polypite meeting with some friction on the glass during its process of retraction, which 
friction must cause it to pull very slightly on its point of suspension. At any rate, if it is due to nervous con- 
nexions between the substance of the polypite and the marginal ganglia, the conducting-power, as estimated by 
the response of the distant tissue, is much less in the direction we are now considering than it is in the converse 
5 E 2 
