660 
MR. GL J. ROMANES ON THE LOCOMOTOR SYSTEM OF MEDUSA. 
bered that excision of the eight marginal bodies of a covered-eyed Medusa is attended with 
the same degree of paralyzing effect as is the excision of the entire margin. It will also be 
remembered that this degree of paralyzing effect is not, generally speaking, so complete 
as it is in the case of the naked-eyed Medusge ; but that, after the operation, occasional 
contractions at long intervals are usually given, and that sometimes, though very rarely, 
these after contractions assume a rhythmical character more or less resembling the 
contractions of the unmutilated animal. All the details concerning the behaviour of 
mutilated gonocalyces in this and in other respects were carefully recorded in my former 
communication. Afterwards, however, I had occasion to add a Postscript to that com- 
munication, in consequence of my attention having been drawn by Dr. Lutken to a 
paper by Dr. Eimer, of the existence of which I was previously unaware. In this 
Postscript I had mainly to deal with certain discrepancies between Dr. Eimer’s account 
of the behaviour of mutilated gonocalyces and that which I had given in my former 
communication. Now, as this want of complete accordance between Dr. Eimer’s 
statements and those which I have published tends to cast doubt upon the accuracy of 
the latter, it becomes necessary for me in the present communication briefly to recur to 
the points wherein Dr. Eimer’s results differ from my own. Of course it is needless to 
say that I have this year given my careful attention to these points, by repeating a 
great number of times the experiments which have reference to them. The following 
statements, therefore, may be considered final, so far as I am concerned. 
The first point of difference to which I must allude is that which is explained in my 
Postscript thus : — “ It will, of course, have been observed that Dr. Eimer’s view as to 
the exact seat of spontaneity in Aurelia aurita does not coincide with mine. He is 
careful to state that the ganglionic function is distributed all round what he terms the 
£ contractile zone,’ i. e. the crescent-shaped interruption of the margin in which the 
lithocyst, together with its gelatinous hood, is situated (see Plate 32). On the other 
hand, I have stated it as my opinion that the lithocyst is alone the locomotor centre ; 
and notwithstanding the account which Dr. Eimer gives of the experiments by which 
he sought to localize that centre, I still adhere to this opinion. Dr. Eimer’s experiments 
in this connexion were twofold : — First, that of progressively lessening the amount of 
contractile tissue left adhering to an excised segment of Aurelia aurita ; and, second, 
that of excising the lithocyst without injuring the ‘ contractile zone.’ Of these two 
methods Dr. Eimer appears to lay most stress upon the first one ; for he merely mentions 
the second method in a short footnote towards the end of his paper, and there states 
that he only tried it in a few cases. Yet to me it seems that for the object in view the 
second method is much more trustworthy than the first. I am well aware of the fact 
pointed out by Dr. Eimer, that upon each contraction of a given segment of Aurelia 
aurita the two arms of the so-called ‘ contractile zone ’ approximate each other, and 
that this gives rise to the appearance of spontaneous action on their part. I think, 
however, that this appearance is deceptive, being caused only by the absence of resist- 
ance at the interrupted part of the margin to the pressure exerted by the contraction of 
