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(e) The third observation will be readily understood from the annexed diagrammatic 
representation of Aurelia aurita (fig. 4). The central circle (x) stands for an open space 
cut out of the gonocalyx ; the outer circle indicates the margin of the animal, with all 
lithocysts save one ( l ) removed ; and the median circular line represents a cut. It will 
be seen that the effect of this cut is almost completely to sever the mass of tissue (y z) 
from the rest of the gonocalyx, the only connexion between them being the narrow neck 
of tissue at y. Yet, in the case to which I refer, the contractile waves emanating from 
l passed in the directions represented by the arrows without undergoing any appre- 
ciable loss of vigour. Upon completing the circular cut at y, the ring of tissue (y z) 
became totally paralyzed, while the outer circle of course continued its contractions as 
before. Now the neck of tissue at y measured only ^ inch across, while the ring of 
tissue y z when cut through and straightened out upon the table measured 1 inch across 
and 16 inches in length. That is to say, 16 square inches of tissue derived its impulse to 
vigorous contraction through a channel inch wide, notwithstanding the latter was 
situated at the furthest point of the circle from the discharging lithocyst which the form 
of the section rendered possible. It should be stated, however, that this experiment 
might be repeated a number of times without yielding a similar result. I regret that 
in this instance I did not try the effect of narrowing this neck of tissue still more ; for, 
from the analogy afforded by numberless observations, I entertain but little doubt that 
some portion of this neck of tissue was pervaded by a very slender though very definite 
line of functionally differentiated tissue ; and that if this line of differentiated tissue 
had not happened to be cut through in the narrowing of the connecting isthmus, the 
width of the latter might have been reduced to a half, a quarter, or even less, without 
impeding the passage of the contractile wave. 
§ 2. (a) Nature of the contractile tissues of swimming-organs. — The last assertion 
leads us to this important part of our subject. Under this heading I propose to treat 
of the question, What is the nature of the general contractile substance of Medusae 1 
Is the contractile tissue of the swimming-organ pervaded by a definite system of sensory 
and motor tracts, so to speak, radiating respectively to and from the marginal centres l 
or is the contractile tissue of the swimming-organ of a more primitive nature, the 
functions of nerve and muscle being more or less blended throughout its substance ? 
Now, for my own part, I deem this question the most interesting one with which the 
present paper is concerned ; for the evolutionist, no less than the physiologist, will 
recognize its importance as of the highest. We have already obtained ample evidence 
of the fact that it is to the Medusae we must look for the first decided integrations of 
tissue having, to say the least, something closely resembling a nervous function to 
subserve; and we have seen that these integrations appear in the form of intensely 
localized centres of spontaneity. It therefore becomes a matter of pressing moment to 
ascertain the manner in which the spontaneous impulses are transmitted from these 
centres and distributed throughout the contractile tissue of the swimming-organ — 
whether a definite system of lines of discharge becomes evolved pari passu with a defi- 
