ME. Gr. J. ROMANES ON THE LOCOMOTOR SYSTEM OE MEDUSiE. 
719 
and that when they do occur they are of so unmistakable a character as to leave 
not the smallest room for supposing them due to any cause other than the passage of a 
stimulus. 
The result of a number of experiments performed in this connexion is to show that 
the severity of section which causes blocking of the tentacular waves varies in different 
individual cases. Very frequently, however, the tolerance of tentacular waves towards 
section is quite as remarkable as is that of contractile waves. Or, adopting our previous 
terminology, section proves that in Aurelia aurita excitational continuity is as difficult 
to destroy as is contractional continuity. 
To economize space, I shall not describe any of the intermediate degrees of tolerance, 
but shall pass at once to the most extreme instance which I have met with. The figure 
(see Plate 31) is one quarter life-size, and was carefully drawn to measurement*. It 
represents a specimen of Aurelia aurita , which, after having had its lithocysts removed, 
was cut in the same way as already described in the case of Tiaropsis indicans (fig. 8). 
The whole band-shaped length of tissue into which the swimming-bell was thus reduced 
was then submitted to the tremendously severe form of section which is represented 
in the figure. Yet on gently stimulating either end of the band-shaped tissue-mass 
a b, a tentacular wave would start from the point of stimulation, and, as represented 
in the figure, course all the way along the margin from end to end of the band-shaped 
massf. 
* i. e. the proportional length of the cuts was so drawn. When the animal is so severely mutilated, the 
swimming-hell floats out in various directions : so that in the Plate the general shape of the bell is to be 
regarded as diagrammatic. 
f [In my MS., as it was originally sent to the Royal Society, there here followed some pages showing the 
confirmation which the above facts supplied to my view that the contractile tissues of Aurelia are pervaded by 
a nerve-plexus, the constituent fibres of which are capable of vicarious action in almost any degree. But as I 
have now, through Mr. Schafer’s collateral work, obtained microscopical demonstration of the presence of such 
a plexus, it becomes unnecessary to adduce the reasoning from function to structure which these pages con- 
tained, and I have therefore struck them out. It seems desirable, however, to quote from these omitted pages 
the following paragraph, which serves to state the present standing of the question as to whether the waves of 
contraction in Aurelia depend for their passage on the muscular elements alone, or likewise upon the nervous 
elements — a question which, it will he observed, is not decided even by the histological demonstration of a 
nerve-plexus : — 
“ But in addition to these general considerations we must remember, more particularly, that in Aurelia aurita 
we have already obtained evidence of more or less distinct conductile tracts— this evidence being even last year 
sufficiently strong to render the plexus theory on the whole the most probable one that could be devised. This 
year that evidence has been further confirmed by explorations by stimuli ; so that the only obstacle in the way 
of our accepting the plexus theory to account for the passage of contractile waves is the enormous amount of 
section which the contractile tissues will endure without these waves becoming blocked. Now the only reason 
why this fact is an obstacle in the way of our accepting the plexus theory is because, upon that theory, this fact 
would require us to suppose the conductile elements to be capable of vicarious action to an almost incredible 
extent. Hence it was that last year the issue with respect to the passage of contractile waves lay between 
