The rate at which contraction-waves traverse spiral strips of Aurelia is variable. It is largely determined by the length and width of the strip; so that the best form of strip to use for the purpose of ascertaining the _maximum_ rate is one which I shall call the circular strip. A circular strip is obtained by first cutting out the central bodies (_i.e._ manubrium and ovaries), and then, with a single radial cut, converting the animal from the form of an open ring to that of a continuous band. I distinguish this by the name "circular" band or strip, because the two ends tend to preserve their original relative positions, so giving the strip more or less of a circular form. Such a strip has the advantage of presenting all the contractile tissue of the swimming-bell in one continuous band of the greatest possible width, and is therefore the form of strip that yields the _maximum_ rate at which contraction-waves are able to pass. The reason why the _maximum_ rate should be the one sought for is because this is the rate which must most nearly approximate the natural rate of contraction-waves in the unmutilated animal. This rate, at the temperature of the sea and with vigorous specimens, I find to be eighteen inches per second. In a circular strip the rate of the waves is uniform over the whole extent of the strip; so that the time of their transit from one point to another varies directly as the length of the strip. But on now narrowing such a strip, although the rate is thus slowed, the relation between the narrowing and the slowing is not nearly so precise as to admit of our saying that the rate varies inversely as the width. The following figure will serve to show the proportional extent to which the passage of contraction-waves is retarded by narrowing the area through which they pass:-- [Illustration: Fig. 13. Time from end to end of a circular strip ... Time after width has been reduced to one-half ... Time after width has been reduced to one-quarter ... Time after width has been reduced to one-eighth ... Fig. 14. In such experiments it generally happens, as here represented, that reducing the width of a circular strip by one-half produces no effect, or only a slight effect, on the rate, while further narrowing to the degree mentioned produces a conspicuous effect. I may also state that if, as occasionally happens, the immediate effect of narrowing a circular strip to one-half is to temporarily block the contraction-waves, when the latter again force their passage, their rate is slower than it was before. It seems as if the more pervious tissue tracts having been destroyed by the section, the less pervious ones, though still able to convey the contraction-wave, are not able to convey it so rapidly as were the more pervious tracts.] [Illustration: Fig. 14.]