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ME. G. J. ROMANES ON THE LOCOMOTOR SYSTEM OF MEDUSJ5. 
nite system of centres of spontaneity, or contractile tissue can atford, so to speak, to 
retain more or less of its protoplasmic nature after spontaneity has become so far deve- 
loped as to be localized in definite centres. In treating of this question, I will only 
resort to theory when compelled to do so by the facts. 
From the observations already detailed it might well be concluded that the method 
of inquiry by section has already settled the question before us, seeing that this 
method has apparently reduced the hypothesis as to the presence of definite lines of 
discharge to an absurdity. A moment’s thought will render obvious how very trying 
the spiral form of section already described (Plate 33) must be to any thing resembling 
a nervous plexus, while a glance at fig. 3 would seem to render the supposition as to the 
presence of any such plexus almost impossible. Nevertheless there is a weighty body of 
evidence to be adduced on the other side. 
(b) Confining our attention to the species which, as already observed, is in every 
respect the best suited for sectional experiments, viz. Aurelia aurita , it is to be observed, 
in the first place, that it is the exception and not the rule for specimens to stand the 
spiral mode of section more than once round the swimming-organ without losing the 
physiological continuity of their tissues. Moreover different specimens exhibit extreme 
variations in their tolerance of this mode of section. Sometimes the contractile wave 
will become blocked when the contractile strip is only an inch long, while in other 
cases, as already stated, the wave will continue to pass through a strip more than thirty 
times that length ; and between these two extremes there are all possible grades of 
tolerance. Now it seems to me that if the tissue through which these contractile waves 
pass is supposed to be of a functionally homogeneous nature, no reason can be assigned 
why there should be such great differences in the endurance of different individuals of 
the same species ; while if we suppose that the passage of these contractile waves is 
more or less dependent on the presence of more or less differentiated lines of discharge, 
we encounter no difficulty ; for it is almost to be expected that in such lowly forms of 
life such lines of discharge, if present, should not be constant as to position. In some 
cases, therefore, it would happen that important lines of discharge would soon be 
encountered by the section, while in other cases it would happen that such lines of 
discharge would escape the section for a longer distance. It is, indeed, almost incredible 
that any one line of discharge should happen to pursue a spiral course twice or thrice 
round the animal, and at the same time happen to be concentric with the course pur- 
sued by the section ; but such an hypothesis is not necessary to account for the facts. 
All we have to assume is that there exists a more or less intimate plexus of such lines 
of discharge, the constituent elements of which are endowed with the capacity of 
vicarious action, and that in some cases the section happens to leave a series of their 
anastomoses in a continuous state. Doubtless even this assumption represents a state of 
things very difficult to believe ; but I do not see that the numberless grades of indivi- 
dual variations already mentioned admit of being accounted for in any other way. More- 
over I think that the difficulty of accepting this explanation will diminish if we cease 
