Another interesting fact brought out by this form of section is, that the radial tubes are tracts of comparatively high irritability as regards the manubrium; for the certainty and vigour with which the manubrium responds to a stimulus applied at one of the severed radial tubes, _f_, _g_, or _h_, _i_, or _j_, _k_, contrast strongly with the uncertainty and feebleness with which it often responds to stimuli applied between any of these tubes. Indeed, it frequently happens that a specimen which will not respond at all to a stimulus applied between two radial tubes, will respond at once to a stimulus applied much further from the manubrium, but in the course of the radial tube _f_ _k_.
And this leads us to another point of interest. In such a form of section, when any part of the mutilated nectocalyx is irritated, the manubrium shows a very marked tendency to touch some point in the tissue-mass _a_ _a_ _d_ _e_ (Fig. 23) by which it still remains in connection with the bell, and through which, therefore, the stimulus must pass in order to reach the manubrium. And it is observable that this tendency is particularly well marked if the section has been planned as represented in Fig. 23, _i.e._ in such a way as to leave the tissue-tract _a_ _a_ _d_ _e_ pervaded by a nutrient-tube _d_ _e_, this tube being thus left intact. When this is done, the manubrium most usually points to the uninjured nutrient-tube _d_ _e_ every time any part of the tissue-band _a_ _l_ is irritated.
Let us now very briefly consider the inferences to which these results would seem to point. The fact that the localizing power of the manubrium is completely destroyed as regards all parts of the bell lying beyond an incision in the latter, conclusively proves, as already stated, that all parts of the bell are pervaded by radial lines of differentiated tissue, which have at least for one of their functions the conveying of impressions to the manubrium. The fact in question also proves that the particular effect which is produced on the manubrium by stimulating any one of these lines cannot be so produced by stimulating any of the other lines. But although these tracts of differentiated tissue thus far resemble afferent nerves in their function, we soon see that in one important particular they differ widely from such nerves; for we have seen that, after they have been divided, stimulation of their peripheral parts still continues to be transmitted to their central parts, as shown by the non-localizing movements of the manubrium. Of course this transmission cannot take place through the divided tissue-tracts themselves; and hence the only hypothesis we can frame to account for the fact of its occurrence is that which would suppose these tissue-tracts, or afferent lines, to be capable of vicarious action. Such vicarious action would probably be effected by means of intercommunicating fibres, the directions of which would probably be various. In this way we arrive at the hypothesis of the whole contractile sheet being pervaded by an intimate plexus of functionally differentiated tissue, the constituent elements of which are capable of a vicarious action in a high degree.