It has been stated more than once already that the vascular lamella of the sensory niche is a part of the lamella that runs round the margin, and so far the only evidence given has been the strip of endodermal fusion running from the marginal lamella to that of the niche. This strip, however, as has been described, does not come to the surface and consequently seems at first sight to be a different structure from the lamella of the margin. That it is not, however, I found very prettily shown in a series of sections of one of my youngest Tripedalia. In this the lamella of the niche as it was traced in successive sections downwards, was found not to form a closed ring at the bottom of the niche, but each side was continued directly and separately downwards to the margin, where it passed into the corresponding part of the marginal lamella. A reconstruction of the condition, similar to that of Fig. 44, is given in Fig. 45, and I think explains itself at a glance. Evidently the vascular lamellæ that connect the lamella of the sensory niche with that of the margin at first come to the surface, like the rest of the marginal system, but as the animal grows older come to lie within the gelatine. In this way the condition found in cross-sections just through the margin of my very small Tripedalia, and represented in Fig. 46, becomes that of the adult seen in the corresponding portion of Fig. 29. It is as complete a demonstration as could be required that the lamella of the sensory niche is at first only a loop of the marginal lamella, a conclusion that had been already reached by H. V. Wilson on theoretical considerations, based upon the facts of the adult structure as he found them in Chiropsalmus.