As was pointed out by Claus, the bottom of the sensory niche—by bottom is meant the vertical wall that separates the space of the niche from the bell cavity—is formed from the subumbrella only. This arrangement of parts, apparently impossible for a structure so far removed from the bell margin as the sensory niche, will be explained more fully under the special topic of the vascular lamellæ, or cathammal plates. It is sufficient at this point to refer to [Fig. 44], which shows the shield-shaped area mapped out by a vascular lamella that connects the endoderm of the stomach pocket with the ectoderm of the bottom of the niche. By this the exumbrella is completely cut off from any part in the formation of the bottom of the niche. Cross and vertical sections through the niche ([Figs. 39 and 37]) help to a better understanding of these relations. Since the base of the stalk of the sensory niche lies within the ring of vascular lamella, the whole organ as well as the bottom of the niche belongs to the subumbrella, and so in spite of its position some distance upwards from the bell margin the sensory club is very properly called a “marginal body” (Randkörper).
The epithelium of the sensory niche consists entirely of the flattened ectodermal surface layer common to the whole exumbrella. No differentiation suggestive of nervous function in addition to that of the sensory clubs can be discovered, although it would be quite natural to expect to find something of the sort, as intimated by Claus (’78, p. 27).
It is worth while to mention again the fact that the eyes are directed inwardly toward the cavity of the bell. The larger and lower of the two median eyes looks into the bell cavity horizontally; the smaller upper eye is turned upward toward the region of the proboscis. This is in the normal pendant position of the sensory club. The stalk, however, is very flexible, and a range of other positions of the sense organs is possible, although nothing was observed to suggest that such positions were within the control of the animal. The eyes evidently have as their chief function to receive impressions of what is going on inside the bell, not outside. Perhaps the strongly biconvex, almost spherical lenses of the median eyes also point to a focus on near and small objects.
5. The Bell Cavity and its Structures. In general, the bell cavity repeats the external form of the bell, being almost cubical. In cross-section it appears very nearly square with the angles in the interradii as seen in the series of drawings that figure sections of the whole jelly-fish at different levels ([Figs. 6-16]). Above, the bell cavity is roofed over by the stomach; below, it is open freely to the water, the opening being narrowed somewhat by the diaphragm-like velarium ([Fig. 3], v); the four flat perradial sides are bounded by the walls of the four broad stomach pockets, to be described when we come to the internal anatomy.
(a) The Proboscis. From the stomach there hangs down into the bell cavity the proboscis or manubrium, which consists of a short funnel-shaped stalk bearing on its distal end the four mouth lobes or lips. The latter are somewhat broadly V-shaped processes lying in the perradii with the convexity directed outwards, and with the concavity on the inside forming the beginnings of four perradial furrows that are continued upwards to the stomach. The four furrows are shown in the stalk of the proboscis in [Fig. 11], which represents a section taken a little above the level of the mouth lobes. The same cross-shaped section of the stalk shows the four perradial prominences or ridges overlying the furrows, which are the direct continuations of the four projecting mouth lobes.
(b) The Suspensoria or Mesogonia. The stomach (leaving out of consideration the proboscis) hangs down into the bell cavity as a slightly sagging saucer-shaped roof ([Figs. 4 and 5]). In the four perradii it is attached to the lateral walls of the subumbrella by four slenderly developed mesentery-like structures, the suspensoria or mesogonia. These are simple ridges of gelatine, covered of course with the epithelium of the bell cavity, which serve to keep the stomach in position much in the way that a shelf is supported by brackets ([Fig. 4], su). The suspensorium accordingly has two parts, curved so as to lie at right angles with each other: a vertical portion lying along the wall of the subumbrella, and a horizontal which passes over from the vertical on to the basal wall of the stomach. In [Fig. 10] the suspensorium in each quadrant is shown cut across just below the angle between the two parts, so that the two appear in the section as projections on the wall of the stomach and on the wall of the subumbrella.
(c) The Interradial Funnels or Funnel Cavities. It will be seen at once that the four suspensoria serve as partitions to divide the upper portion of the bell cavity, the part that lies between the stomach and the lateral walls of the subumbrella, into four compartments. These compartments extend upwards in the four interradii like inverted funnels, whence their name. In the series of cross-sections they can be traced upwards with constantly diminishing area from the level of the suspensoria, [Fig. 10] (if), to [Fig. 6], which is taken very near the top of the bell. Homologous structures exist in all the Scyphomedusæ, and in some of the Lucernaridæ they are continued up even into the stalk of the attached jelly-fish.
(d) The Velarium. Charybdea, like most of the Cubomedusæ, possesses a velum-like structure around the opening of the bell cavity ([Fig. 3], v). The velarium is a thin muscular diaphragm, resembling the true velum in position and essential structures, but differing from the velum in its origin, and in the possession of diverticula from the gastro-vascular system, the velar canals. Of these there are in C. Xaymacana very regularly sixteen, four in each quadrant. Their outline is seen in [Fig. 3] to be forked with small irregular accessory processes. As for its origin, the velarium of the Cubomedusæ is commonly accounted to have arisen by fusion of marginal lobes, as in the case of the velarium of the Discomedusæ. Pending decisive ontological evidence, the slight notches in the four perradii seen in [Fig. 3] may perhaps be taken as slight indications of a primitive unfused condition, but the question will be brought up again when the vascular lamellæ are discussed.
(e) The Frenula. Just as the stomach is attached to the walls of the subumbrella in the four perradii by the suspensoria, so in the lower part of the bell cavity the velarium is attached to the wall of the subumbrella in the perradii by four structures similar to the suspensoria, the frenula velarii. The frenula, like the suspensoria, resemble the brackets of a shelf, with the difference that in the case of the frenula the bracket is above the shelf, their purpose being evidently to keep the velarium stiff against the outflow of water produced by the pulsations of the bell. According to the greater need of strength in this case, we find the frenula stouter, more buttress-like than the suspensoria. The gelatinous ridge that gives them the necessary firmness is thickened so as to be triangular in section, as shown in [Fig. 16] (frn).
(f) Musculature. As is general in medusæ, the muscular system, so far as known, is restricted to the subumbrella. It has a very simple arrangement, consisting of a continuous sheet of circular (i. e. horizontal) striated fibres, which is interrupted only in the four perradii by the radially directed muscle fibres of the suspensoria and the frenula. In each quadrant, between the muscle of the suspensorium above and that of the frenulum below, in an area just internal to the sensory niche, there lies a space free from muscle. This interruption of the muscle layer is shown in [Fig. 39]. Under the head of musculature belonging to the subumbrella must be included also the radial, or longitudinal muscles at the bases of the pedalia, which were mentioned before ([Fig. 16], mt). The mouth lobes and proboscis also are highly contractile and muscular.