The Cubomedusæ

by Franklin Story Conant

A: The Vascular Lamellæ.

In Medusæ it is a common thing to find that in certain definite places of the gastro-vascular system two endodermal surfaces that were primarily separated by a space have come together and fused into a single lamella or plate. Such a structure is called indifferently a cathammal plate, an endodermal lamella, or a vascular lamella. In the adult animal the vascular lamellæ are by virtue of their very nature formations “with a past.” They are scaffolding left in the completed structure, giving us clues as to the way in which that structure was brought about; and in the Cubomedusæ, whose development is as yet unknown, they therefore afford an unusually interesting subject for special consideration.

The vascular lamellæ that are found in Charybdea and Tripedalia may for convenience be described as forming two systems, the internal and the marginal. The former comprises the endodermal fusions that separate the stomach from the stomach pockets (except for the spaces of communication left free, the gastric ostia) and those that separate the stomach pockets from one another. The marginal system consists of the lamella that connects endoderm of the gastro-vascular system with ectoderm of the surface in a ring all around the bell margin, and with it also the vascular lamella of the sensory niche, which has already been referred to in the general description of Charybdea. The lamellæ of the internal system have been described by previous writers, and especially by Claus in his paper on Charybdea, but they are still in need of comprehensive and clear treatment. The lamellæ of the margin and of the sensory niche have also been described by Claus, but not thoroughly or with entire accuracy, nor did he recognize the vascular lamellæ of the sensory niche as originally a part of the lamellæ of the margin. This last was first determined by H. V. Wilson upon specimens of Chiropsalmus quadrumanus obtained at Beaufort, North Carolina. Professor Wilson’s unpublished notes on Chiropsalmus were very kindly placed in my hands, and so far as the vascular lamellæ are concerned my own work is only a confirmation and amplification of his, since Charybdea and Tripedalia in this respect agree with Chiropsalmus.

The vascular lamellæ of the internal system are the most prominent and morphologically the most important. They comprise the four vertical strips of fusion that separate the four stomach pockets in the interradii (ivl in the figures of the series of cross-sections of Charybdea and Tripedalia, Nos. [6-15] and [21-29]), and four curved horizontal cross-pieces at the top of these which separate the stomach from the stomach pockets, and would make the separation complete did they not leave in each perradius a free space between their ends, which makes possible the gastric ostia.

The arrangement of this internal system of vascular lamellæ is simple. What they amount to is a certain definite number of linear adhesions between the two walls of an originally undivided gastro-vascular space, by which that space is divided up into a central stomach and a peripheral portion, and the peripheral portion thus further divided into the four stomach pockets. Perhaps the idea may be conveyed by likening the whole medusa to a couple of bowls fitting closely one within another and plastered together at the margins. The exumbrella then would correspond to the outer bowl, the subumbrella to the smaller inner bowl, and the original undivided gastro-vascular space to the space between the two. If now the walls of the space be cemented together in four horizontal curved lines just in the plane where the bottoms are bending round to become the sides of the bowls, leaving four interspaces between the ends of the lines, we should have the original space divided into a central horizontal somewhat lens-shaped region between the bottoms of the two bowls that would correspond to the central stomach, and a peripheral vertical portion between the sides of the bowls that would correspond to the peripheral gastro-vascular system; central and peripheral portions would communicate by the four interspaces between the lines of fusion, which would correspond to the four gastric ostia. If, further, the vertical peripheral portion be subdivided by four more lines of fusion running vertically at equal distances apart, each connecting above with the middle point of the corresponding horizontal line of fusion, we should have the simple peripheral portion divided into four parts, corresponding to the stomach pockets, by four vertical lines of fusion, corresponding to the four interradial vascular lamellæ, the ivl of the figures.

These mutual relations of stomach, stomach pockets and lamellæ will perhaps be made clearer if a comparison is drawn between them and the similar structures of a Hydromedusa. Liriope, one of the Trachomedusæ, is a good form to take for such a comparison, since by reason of its direct development from the egg it is free from the complications of hydroid medusæ. The young medusa has at first a simple, undivided gastro-vascular cavity which later is divided up into the central stomach and the typical radial to circular canals of the Hydromedusæ by means of fusions between the two endodermal surfaces. Diagrams a, b and c of [Fig. 35] represent very schematically this process of division into stomach and canals. In a we have a projection upon a plane surface of the primary, undivided gastro-vascular cavity, as seen from above; b shows the first four points of fusion in the interradii; c represents those four points expanded by growth in all directions into broad cathammal plates in such a way as to leave the stomach in the centre, the radial canals in the perradii, and the circular canal in the periphery as all that remains open of the primary simple cavity. These broad plates of vascular lamella, separating the narrow radial canals, persist in the adult Liriope to tell the tale of the formation of the definitive gastro-vascular system. It seems to me that we are justified by analogy in drawing a similar conclusion for the Cubomedusæ. In d of [Fig. 35] is represented a projection of a Cubomedusa, in which the homology of the stomach pockets with the radial canals of the Hydromedusa, and of the narrow strips of fusion with the broad cathammal plates, is shown at a glance. To make the comparison more perfect we have only to remember that in the Cubomedusæ there exists below each interradial vascular lamella a connecting canal (Figs. [16], [29] and [35] d, cc) uniting the two separate adjacent pockets. This, as has been pointed out by other writers, is the representative of the circular canal of the Hydromedusæ. Practically the only difference between the structure of the gastro-vascular system of the Cubomedusæ and that of a form such as Liriope, is that in the latter the fused areas have broadened out at the expense of the radial canals, while in the Cubomedusæ on the contrary they have become long and narrow.

One is strongly tempted by the foregoing comparison to speculate a little as to whether the reproductive organs of the Cubomedusæ, which lie in the stomach pockets and are generally supposed to be endodermal, may not bear some closer relation to those of the Trachomedusæ, which lie “in the course of” the radial canals (Lang’s Text-book) and by common consent are ectodermal. And while we are being led by facts such as those just mentioned above to wonder just a little whether after all the position of the Cubomedusæ among the Acraspeda is so firmly assured—doubting some, yet in the frame of mind of one who “fears a doubt as wrong”—the velarium suggests itself as another point in question. Haeckel does not hesitate to state emphatically that the velarium of the Cubomedusæ and the velum of the Craspedote medusæ are only analogous, but the reasons that he gives (sie sind unabhängig von einander entstanden, und ihre Structur ist zwar ähnlich, aber keineswegs identisch; namentlich das Verhalten zum Nervenring ist wesentlich verschieden: System, p. 426) somehow do not produce so much impression upon one as the very velum-like appearance of the velarium itself. The origin from the fusion of marginal lobes is not as yet a matter of observation, and the relation to the nerve ring is not essentially different from that of the velum to the lower (i. e. inner) nerve ring in the Craspedotæ. The four frenula and the diverticula from the gastro-vascular system seem to be the chief differences in structure after all, and these Haeckel evidently did not think worth mentioning. This speculation, as to the possible relation of the Cubomedusæ to such forms of the veiled medusæ as Liriope, though it may be very tempting, is scarcely fruitful enough to repay much effort on the part of either reader or writer. The whole subject must remain uncertain until the facts of the development of the Cubomedusæ are known.

If the structure of the vascular lamellæ of the internal system has been made clear, the appearances of the vertical and horizontal components in the figures will be understood without much further explanation. The four vertical strips in the interradii (ivl) have been already referred to in the figures of the cross-sections of both Charybdea and Tripedalia. In the longitudinal sections of the two jelly-fish through the interradii, the vertical lamellæ are cut throughout their entire length from stomach to connecting canals (Figs. [5-20], ivl). The horizontal cross-pieces at the tops of the vertical lamellæ also appear in several of the figures. [Fig. 36] represents the appearance that would be given by a longitudinal section taken through any portion of the upper part of the bell except in the interradii, or in the perradii, through the gastric ostia. The horizontal vascular lamella (hvl) is shown connecting the endoderm of the stomach (ens) with that of the stomach pocket (enp). In a longitudinal section directly through an interradius ([Fig. 5 or 20]) the horizontal lamella is cut just at the point where it joins the vertical, so that the two are not differentiated. In a section through the region of a perradius ([Fig. 4 or 19]) the horizontal lamella is of course not cut, since the section passes through the gastric ostium, whose existence is conditional upon fusion not having taken place between the endodermal surfaces.

The first figure in each of the series of cross-sections (Figs. [6] and [21]) also shows the horizontal vascular lamella, cut across slantingly twice in each quadrant as it passes between the gelatine of the ex- and of the subumbrella to connect the epithelium of the stomach with that of the stomach pocket. The fact that more of the lamella does not appear in such a cross-section only shows that its course is not perfectly horizontal.

The region in which the same lamella lies is indicated in the surface view of the top of the bell of Charybdea ([Fig. 2]) by the bent line hvl in each quadrant. The figure manifests the appropriateness of Claus’s name for the horizontal lamella—“bogenförmige Verwachsungs-Streifen.” Haeckel calls the same structures “Pylorus-Klappen,” and in his account of Charybdea Murrayana in the Challenger Report, speaking of the three divisions of the stomach (buccal, central and basal) which he traces upwards from the stalked forms of Scyphomedusæ, he says: “The central stomach in this Charybdea, as in most Charybdea, is joined to the basal stomach, as the pyloric stricture between the two is not developed and only faintly indicated by the slightly projecting pyloric valves.” Again, in speaking of the valves of the gastric ostia, he says: “These four perradial ‘pouch valves’ alternate with the interradial pyloric valves.” It is difficult to understand, however, how the “bogenförmige Verwachsungs-Streifen” of Claus, which are undoubtedly the same structures as those which I have called the horizontal lamellæ, and are only strips of endodermal fusion, can be “projecting pyloric valves,” or indeed can properly be spoken of as valves at all. Possibly Haeckel was not quite able to understand Claus’s description, and in his desire to find something in the stomach of Charybdea which would serve to set off a central from a basal part, such as is found in the Lucernaridæ, hit upon Claus’s “Verwachsungs-Streifen.” I have elsewhere given it as my opinion that in such of the Cubomedusæ as I have studied there is no structure in evidence that would properly serve to mark a limit between a basal and a central portion of the stomach.

We have next to describe the marginal system. The vascular lamellæ mentioned above in every case connected endoderm of one cavity with endoderm of another; those of the margin have the noteworthy difference that they run from endoderms of some part of the gastro-vascular system to ectoderm of the surface. The outermost cells of the endodermal lamellæ make direct connection with the ectodermal cells, without the usual intervention of a layer of gelatine.

The marginal lamella of Charybdea lies, as the name implies, just on the bell margin where the edge is curving round into the velarium. All around the whole circumference of the bell it is found (in Charybdea) at this same horizontal bend, except in the eight principal radii, where the tentacles and the sensory clubs have brought about modifications. In any place except these a vertical section through the margin will show the marginal lamella connecting the endoderm of the marginal pocket with the ectoderms of the surface, as represented by vlm in [Fig. 38], which is a vertical section through the sensory niche a little to one side of the perradial axis.

In the interradii the marginal lamella undergoes modifications due to the fact that the bases of the pedalia are situated a little upwards from the exact margin, and that the lamella follows the outline of the bases. [Fig. 1] shows one of the interradial corners of the bell margin looked at directly from the surface, so that the curved outline of the junction of the base of the pedalium with the exumbrella is seen. The trace made by the lamella where it meets the surface ectoderm follows this outline. The lamella is also shown in the vertical section through the interradius ([Fig. 5 or 20], vlm), where it is seen running from the connecting canals (cc), which joins the two adjacent marginal pockets, upwards and outwards to meet the surface ectoderm. Its course from canal to surface is not in a direct line, but curved with the concavity upwards. Hence, in cross-sections at certain levels through the interradial corner it is met more than once and gives rise to appearances that seem at first sight too complicated for it to be just the same structure as the simple marginal lamella described above. That it is the same, and that the complication is only due to the insertion of the pedalia above the margin, can be determined by following through a series of cross-sections, the essential ones of which, as I hope, are given in [Figs. 40-43]. The levels of these are shown on [Fig. 5] by the letters w, x, y and z, respectively. [Fig. 40] shows the lamella cut but once, just below its highest part. The section is above the level of the connecting canal and hence still shows the vertical interradial lamella ivl. [Fig. 41], at the next lower level (x), shows the same portion of the lamella intersected a little nearer the interior, while the junction with the endoderm of the connecting canal is shown still further inside. [Fig. 42] is at level y, just through the bend of the loop, so that in part of its course the lamella is cut almost horizontally, i. e. in its own plane. [Fig. 43] finally shows the lamella as it appears below the level of the connecting canal, cut twice, each portion joining endoderm of marginal pocket with ectoderm of surface. It thus bears exactly the same relations that it had when we first met it in [Fig. 38] (vlm), except that here in [Fig. 43] one finds that a cross-section cuts it at right angles instead of a vertical as in [Fig. 38], as a result of its being pushed upwards from its former position on the margin by the insertion of the pedalium above the margin.

The vascular lamella of the sensory niche has already been alluded to as part of the marginal system, and brief reference has been made to it in the section on the sensory clubs. Like the rest of the marginal lamella, it connects endoderm with ectoderm. The line that its fusion with the ectoderm traces on the surface frames in a shield-shaped area at the bottom of the sensory niche, which is seen in the drawing of the outlines of the niche, [Fig. 44] (vls). This lamella was observed by Claus, and was figured by him both in surface view and in cross-section through the niche. Apparently, however, he omitted vertical sections through the niche, so that he supposed that the outline traced by the lamella was not continuous above, i. e. over the stalk of the sensory club (’78, Fig. 41; text, p. 28). That the outline is closed above, though masked in surface view by the roof of the sensory niche, is seen at once in vertical sections, such as [Figs. 37 and 38], one of which is directly through the perradius, the other a little to one side. Both show the vascular lamella of the sensory niche (vls) intersected twice, above and below the sensory club, and completely cutting off the exumbrella from any share in the bottom (or inner wall) of the sensory niche. [Fig. 39], which is a cross-section through the upper part of the niche, and is essentially like the similar figure of Claus, shows in like manner that the bottom of the sensory niche belongs to the subumbrella. H. V. Wilson was the first to point out, in his unpublished notes, that the lamella of the niche is complete all round.

In the adult structure of Charybdea and Tripedalia the lamella of the niche is connected with that of the margin by a vertical strip of endodermal fusion that does not come to the surface like the rest of the marginal system, but remains just internal to the gelatine of the exumbrella, connecting the two adjacent marginal pockets. In the cross-sections of Charybdea it is seen in [Fig. 16] (vlc); in those of Tripedalia it is seen in [Figs. 28 and 29]. In vertical section it is found in Figs. [4], [19] and [37]. In [Fig. 44], which represents the bell margin and velarium of Tripedalia arranged as if the velarium were vertical and pendant from the margin (instead of suspended by the frenulum so as to be at right angles to the vertical plane), the connecting lamella is shown as a dotted line (vlc)—dotted because it does not come to the surface—joining the lamella of the niche with that of the margin (vlm).

The same figure ([No. 44]) shows a characteristic difference between the marginal lamella of Tripedalia and that of Charybdea. While in Charybdea, as Claus points out, the marginal lamella keeps at one level, just a little above the bell margin, all the way round (except where disturbed by the special modifications of the tentacles and the sensory clubs), and never descends into the velarium, in Tripedalia on the other hand it describes a sinuous course, following the outlines of the marginal pockets, as is indicated in the figure by the light parallel line vlm. The course as it would be seen in a surface view is obscured just at each side of the interradius by the overhanging of the bases of the two lateral pedalia. This is why the lamella is not indicated at these points in the diagram. The course is seen to lie almost wholly on the velarium, that is, in the figure below the line which represents the bell margin proper, the line at which the angle comes when the velarium is in its normal position, horizontal to the vertical side of the bell.

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.