MR. Gr. J. ROMANES ON THE LOCOMOTOR SYSTEM OE MEDUSAE. 
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a ganglionic swelling, and it is from these ganglionic swellings that the nerves just 
mentioned take their origin. The most conspicuous of these nerves are those that 
proceed to the radial canals and marginal vesicles, while the least conspicuous are those 
that proceed to the tentacles. Cells, as a rule, can only be discerned in the ganglionic 
swellings, where they appear as fusiform and distinctly nucleated bodies of great trans- 
parency and high refractive power. On the other hand, the nerves that emanate from 
the ganglia are composed of a delicate and transparent tissue, in which no" cellular 
elements can be distinguished, but which is longitudinally striated in a manner 
very suggestive of fibrillation. Treatment with acetic acid, however, brings out distinct 
nuclei in the case of the nerves that are situated in the marginal vesicles, while in those 
that accompany the radial canals ganglion-cells are sometimes met with. 
It will be seen from this brief resume that Prof. Haeckel’s account of the distribution 
of the nervous tissues in Medusae closely resembles that which had already been given 
by Agassiz, and from this it might be supposed that the one series of observations are 
confirmatory of the other. Prof. Haeckel himself, however, is clearly of a very different 
opinion, and in several essential points his description certainly does not agree with 
that of Agassiz. For my own part, as I am able conscientiously to endorse all that 
Prof. Haeckel says with regard to the extreme difficulty of distinguishing, in any of the 
species named by Agassiz, histological elements to which a nervous function might 
reasonably be assigned, I cannot help still thinking that the last-named observer must have 
been mistaken in his inferences concerning the tissue, which he says he observed 
without subjecting it to treatment of any kind. Possibly the structure which he describes 
may have been the optical expression of the thickness of the ectoderm in the region of 
the nutritive canals. Be this as it may, however, I cannot allow this postscript to appear 
without stating that in my opinion the microscopical researches of Prof. Haeckel in 
this connexion are of much greater value than those of any previous observers ; and this 
not only because of his deservedly high reputation as an histologist, but also because, 
judging from his figures, the tissues to which he ascribes a nervous function bear so 
close a resemblance to nervous tissue in general. Moreover, while this experienced 
microscopist is careful to state that even in the Geryonidce it is no easy matter to distin- 
guish the nervous elements, he also states (as already observed) that in this group these 
elements are much more readily distinguishable than in any other group of the naked- 
eyed Medusae. For these reasons, therefore, I consider Prof. Haeckel’s deductions from 
anatomical structure to physiological function as of a more legitimate, and so of a more 
trustworthy, character than were those of Prof. Agassiz ; and when they are taken in 
conjunction with the remarkable verification which they receive from the experiments 
to be detailed in this memoir, they ought, I think, to be regarded as finally decisive 
of the long-disputed question as to the presence of a nervous system in Medusae.] 
§ 2. Movements of the Medusae . — It is of course known to every one that the Medusae 
are naturally locomotive animals, the various species swimming more or less rapidly by 
means of an alternate contraction and dilatation of the entire swimming-organ. It may 
2 Q 2 
