The Cubomedusæ

by Franklin Story Conant

In this sinuous course of the marginal lamella we have another point of resemblance between Tripedalia and the Chirodropidæ. H. V. Wilson worked it out in his sections of Chiropsalmus, and the reconstruction which I have given in the figure under discussion is in all essentials similar to his for Chiropsalmus. The differences lie only in the fact that Chiropsalmus has more velar canals, and that the chief marginal pocket in each quadrant is not forked peripherally, as is that of Tripedalia (mp), but presents its distal margin parallel to the edge of the velarium. The two smaller marginal pockets in the perradii (mp´) are on identically the same plan in both.

Tripedalia, having three tentacles joining the umbrella in each interradius, shows a disturbance of the course of the marginal lamella in these regions by just so much the more complicated than in Charybdea. The plan, however, is exactly the same. The lamella is pushed upwards from the margin by each of the bases of the three pedalia just as is done by the base of the single pedalium of Charybdea. [Fig. 29] shows the lamella in the same relation to the canal of the central tentacle (ct) that it has in the similar sections of Charybdea (Figs. [16] and [43]); and in addition the first appearances (as the series is traced downwards) of the arches of the lamella over the two lateral tentacles (ct´), which are inserted a little lower down than the middle one of the group. As concerns these lateral tentacles, the relations of the vascular lamella at this level are the same as that in the level of [Fig. 40] for Charybdea.

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.

As Wilson pointed out in his notes, these facts have a close bearing upon the question of the origin of the velarium. Sixteen marginal pockets are found in both Chiropsalmus and Tripedalia, and all of them extend into the velarium. It is not unnatural to suppose that these belong to sixteen marginal lobes, and that these lobes have fused together to form the velarium. In the Chirodropus figured by Haeckel (Taf. XXVI) in his “System” gelatinous lobe-like thickenings are shown in the velarium, corresponding to the sixteen marginal pockets. In Tripedalia no special gelatinous thickenings are found, but the arrangement of the marginal pockets is the same as that of the Chirodropidæ, and perhaps I ought, when treating of the systematic relations of Tripedalia ([p. 5], Fam. III), to have recognized the analogy to the extent of saying that marginal lobes may not be completely absent from the velarium of Tripedalia. At any rate the gelatinous lobes in the case of Chirodropus on the one hand, and on the other hand the sinuous outline of the margin still mapped out by the lamella in Chirodropus, Chiropsalmus and Tripedalia, are certainly very suggestive of an ancestral Cubomedusa in which there was no velarium, but sixteen free marginal lobes instead. Two more indications favor slightly the same view. In both Charybdea and Tripedalia a small notch is seen in the edge of the velarium in the perradius ([Fig. 44]). Its constancy suggests that it may not be a chance or meaningless feature. The second point is the small size of the two marginal pockets adjoining the perradius. These are in the position of the ephyra lobes of the Discomedusæ, which always lie on either side of each sensory club, and which do not keep pace with the other marginal lobes in development. In the Rhizostome jelly-fish especially they are found much smaller than the other lobes, as will be seen by a glance at such figures as Haeckel’s for Lychnorhiza (System, Taf. XXXIV Fig. 2), or for Archirhiza (Taf. XXXVI, Fig. 5), or Hesse’s figure of the margin of Rhizostoma Cuvieri (’95, Taf. XXII, Fig. 22). The resemblance between such margins and that of Tripedalia ([Fig. 44]), with its simple, unbranched velar canals, is very suggestive. On the other hand it must be remembered that in considering the vascular lamellæ of the internal system we found the indication pointing rather more to Hydromedusan affinities than to any other. Charybdea throws no light on the question, since it has no marginal lobes on the velarium and the marginal pockets end strictly at the margin, so that the only diverticula of the gastro-vascular system in the velarium are the velar canals.

Before leaving the subject of marginal lobes and pockets I must answer a possible objection that may occur to some careful reader. It may seem that I am wrong in holding that there are two marginal pockets in each octant instead of three, that just as there is one velar canal from each of the smaller perradial pockets (mp´, [Fig. 44]), so each prong of the forked larger pocket (mp), since it is continued into a velar canal, ought to be called a marginal pocket likewise, the whole number of marginal pockets then being twenty-four instead of sixteen. Such a revision of the terminology would not be without some reason in its favor, and perhaps a study of more forms would show it to be correct. But for the present, at any rate, it seemed to me best to abide by the analogy of Chiropsalmus, in which the peripheral edge of the larger marginal pocket in each octant is not bow-shaped, but runs parallel to the edge of the velarium. A revision of the terminology of the marginal pockets such as implied in the suggestion above would also give rise to complications when applied to Charybdea, since the latter has no marginal pockets in the velarium.

As to the functions of the vascular lamellæ, there is too little known to say much. It is rather improbable that structures retained so definitely should be mere scaffolding left over from a previous stage of usefulness. Claus has found in Chrysaora that the lamellæ form a kind of capillary network in communication with the gastro-vascular system, and he with others supports the view that they perform an accessory function in the nutrition of the tissues they penetrate. Upon this point I have no observations of my own to add.

The marginal vascular lamella is regarded by Claus as perhaps the vestige of a circular canal around the bell margin. On this subject, too, I have nothing to add. A lamella of endoderm that connects directly with the ectoderm of the surface along its whole course is a structure whose meaning I am wholly unable to understand or even to guess at. A similar lamella is described by Hesse (’95, p. 430) as occurring in the ephyra lobes of his Rhizostoma, and he mentions Eimer as the first to discover this structure, probably meaning the first to discover it in the Discomedusæ. Whether the lamella is found all around the margin is not stated. Hesse refers it to the ephyra, and remarks that the investigation of it in the ephyra would undoubtedly give interesting results.

I will close this part upon the vascular lamellæ with a very pertinent suggestion made by Professor Brooks to the effect that the usual way of speaking of the sensory clubs as having moved up from the margin is looking at the matter in the wrong way. The level of the sensory clubs undoubtedly represents the original margin, which elsewhere has grown down and away from its former level, leaving the sensory clubs like floatage stranded at high-tide mark. Only in this way can the lamella of the sensory niche have any meaning.

B: The Nervous System.

The nervous system of the Cubomedusæ is the most highly developed that is found in any of the jelly-fishes. If the position of the group among the Acraspeda is established, it alone is ample to prove that the Hertwigs had not sufficient evidence when they stated in their monograph on the nervous system of the Medusæ (’78) that the Acraspeda show a much lower nervous organization than the Craspedota.