The manubrium is the mouth and stomach of the animal, and at the point where it is attached to or suspended from the umbrella its central cavity opens into a tube-system, which radiates through the lower or concave aspect of the umbrella. This tube-system, which serves to convey digested material and may therefore be regarded as intestinal in function, presents two different forms in the two main groups into which the Medusæ are divided. In the "naked-eyed" group, the tubes are unbranched and run in a straight course to the margin of the umbrella, where they open into a common circular tube which runs all the way round the margin (see Figs. 1 and 22). In the "covered-eyed" group, on the other hand, the tubes are strongly branched (see Fig. 8), although they likewise all eventually terminate in a single circular tube. This circular or marginal tube in both cases communicates by minute apertures with the external medium. The margin of the umbrella, both in the naked and covered eyed Medusæ, supports a series of contractile tentacles, which vary greatly in size and number in different species (see Figs. 1 and 8). The margin also supports another series of bodies which will presently be found to be of much importance for us. These are the so-called "marginal bodies," which vary in number, size, and structure in different species. In all the covered-eyed species these marginal bodies occur in the form of little bags of crystals (therefore they are called "lithocysts"), which are protected by curiously formed "hoods" or "covers" of gelatinous tissue; and it is on this account that the group is called "covered-eyed," in contradistinction to the "naked-eyed," where these little hoods or coverings are invariably absent (compare Fig. 1 with Fig. 22), and the crystals frequently so. In nearly all cases these marginal bodies contain more or less brightly coloured pigments. The question whether any nervous tissue is present in the Medusæ is one which has long occupied the more or less arduous labours of many naturalists. The question attracted so much investigation on account of its being one of unusual interest in biology. Nerve-tissue had been clearly shown to occur in all animals higher in the zoological scale than the Medusæ, so that it was of much importance to ascertain whether or not the first occurrence of this tissue was to be met with in this class. But, notwithstanding the diligent application of so much skilled labour, up to the time when my own researches began there had been so little agreement in the results obtained by the numerous investigators, that Professor Huxley--himself one of the greatest authorities upon the group--thus defined the position of the matter in his "Classification of Animals" (p. 22): "No nervous system has yet been discovered in any of these animals."