The Cubomedusæ

by Franklin Story Conant

[Fig. 51] is a vertical section through the base of the stalk in the plane of the reference arrow w-z in [Fig. 49], and therefore passing through one of the roots of the nerve of the stalk. Here again the region is seen to be cut off from the exumbrella by the vascular lamella of the sensory niche (vls), and the nerve is seen passing through the gelatine of the subumbrella from the surface of the bell cavity (sc) to the base of the stalk hanging in the sensory niche (sn). One of the ganglion cells (gc) that accompany the nerve is seen to have two nuclei, a not infrequent occurrence which has been pointed out by others.

The same figure shows that the axis (ax) of the nerve has penetrated the gelatine with the other fibres. Here at the base of the stalk it takes a horizontal course and becomes directly continuous with the similar structure of the other root, as Wilson, I believe, first pointed out. This part of the nervous tract which runs horizontally along the base of the stalk between the two roots ([Fig. 49], rns) has been considered by Claus the representative in Charybdea of the upper nerve ring of the Craspedota, which therefore exists in Charybdea in four separate portions. Seeing, however, that the region in which it is found belongs to the subumbrella, the homology seems very doubtful. Moreover, the fact that the axis of the nerve ring runs through this outer portion, instead of remaining on the inner surface of the subumbrella and passing to the radial ganglion, rather indicates that the outer portion is part of the original course of the nerve ring, while the portion that remains on the inner surface is perhaps a later formation.

A very interesting feature of the nervous system occurs in the same region in the form of a tract of fibres underlying the endoderm, and separated from the other fibres by the gelatine of the supporting lamella. It is seen in vertical section in [Fig. 52] (enf), which is a section through the base of the stalk in just about its median plane, and, therefore, to one side of the arrow w-z in [Fig. 49] and the corresponding drawing, [Fig. 51]. In cross-section it is represented also in [Fig. 50] (enf). It varies in size and prominence very much in different specimens. [Fig. 52] is a camera drawing of it in the case that showed it most developed. Ganglion cells are found in it, but comparatively infrequently. In some cases the tract itself can hardly be found with certainty. Hesse has described in a Rhizostome a much more highly developed tract in a corresponding position on the base of the marginal body. Fibres from the “outer sensory pit” pass through the gelatine to the sub-endodermal tract, which is described as surrounding the epithelium of the canal of the marginal body like a collar and is most thickly developed on the under surface of the canal, at the place that just corresponds with the point where, and where only, I find the tract in Charybdea. Hesse thinks that fibres then pass from this region to the nervous epithelium of the “inner sensory pit” lying underneath the base of the marginal body, which contains a rich supply of ganglion cells and is considered by him to be the centre of the nervous system of the medusa. A close comparison cannot be drawn with Charybdea in this matter, however, since Charybdea has nothing to correspond with the “outer” and “inner” sensory pits. Moreover, the endodermal tract is not found encircling the canal of the sensory club, nor could I trace fibres passing from it through the supporting lamella into the fibres of the nerves.

Claus has figured (’78, Taf. V, Fig. 45, Fb) a small bundle of fibres in the stock of the sensory club lying between the endoderm cells of the canal and the supporting lamella. The same bundle is found in both Charybdea and Tripedalia and can be traced in cross-sections up the stalk to a point which must correspond with that at which the endodermal tract is seen in [Fig. 52]. Downwards it can be traced only as far as the entrance of the stalk into the knob of the club where it invariably becomes lost to view. According to Hesse (’95, p. 427) Schäfer found under the endoderm cells of the whole stalk of the marginal body a fibrous layer like that under the endoderm cells which he refers to slender processes from the cells of the crystalline sac. Although Hesse, as we have seen, finds the layer more limited in extent than Schäfer gives it, and does not trace it to the same source, the observation of Schäfer seems to me worthy of mention here, inasmuch as the trend of the fibrous bundle under the endoderm cells of the stalk in Charybdea and Tripedalia suggests quite strongly that the fibres come from the crystalline sac, as Schäfer thought to be the case in his medusa.

Besides the radial ganglion situated in the course of the nerve ring at its four perradial points there are four other similar ganglia on the subumbrella. These lie in the interradii, at the four lowermost points of the nerve’s course, and undoubtedly send off nerves into the pedalia at whose bases they are situated. F. Müller (’59), whose work was not accessible to me, is quoted by Claus as recording two ganglia opposite the base of each pedalium which gave off a great number of nerves partly into the velarium, partly into the tentacles. Claus observed nothing of the kind in Charybdea and states that even the interradial ganglia do not exist.

That they do, however, is shown without doubt in sections of both C. Xaymacana and Tripedalia, but nerves to the velarium or to the tentacles I was unable to find.

On the two sides of each frenulum and of each suspensorium are found subepithelial ganglion cells in greater numbers than elsewhere on the subumbrella, and I am inclined to ascribe to them also the importance of special ganglia controlling the musculature of the frenula and suspensoria. Certainly such ganglia would not be out of place.

It has been mentioned that the greater prominence of the radial nerve and the possession of special sensory organs in the proboscis were the only points of difference I had noted between the nervous systems of Charybdea and Tripedalia. These sensory organs remain to be described. They are simple ciliated cysts containing a concretionary mass, and are situated in the gelatine of the proboscis, irregularly disposed of at any level, from the lips to the beginning of the stomach, and in any radius. In one series of the adult animal fifteen were counted, of which seven were situated about interradially, four perradially, two adradially and two subradially. In another, twenty-one were counted, twelve in the perradii and nine situated between the sub-and perradii. The one shown in [Fig. 24] is in the perradial position, often seen. In the sections of the very young Tripedalia in which the vascular lamella had not reached the adult condition the sensory organs of the proboscis were not found, although the sensory clubs showed practically no difference from the adult. Their structure is very simple—merely a round or oval sac lined with ciliated cells which bear up and keep in constant motion an irregular coarsely granular concretion. [Fig. 53] is a sketch made in Jamaica from the living specimen. Sections were somewhat disappointing in that they added but little. [Fig. 55] was drawn to show that now and then a mucous cell (mc) is found among the other cells of the sensory epithelium. An irregular-shaped mass (rc) was always found inside the cysts as the organic remains of the concretion. It gave no trace of cellular structure and offered no evidence whether the concretion was the product of one or few or of all the cells of the cyst. The latter would be unique among the medusæ. Even if the otocyst is the result of the activity of only one or a few cells, it is, so far as I know, the only case known for the jelly-fish of a free, unsuspended concretion.

As to whether the cysts are of ectodermal or endodermal origin could not be determined, but there was some evidence in favor of the latter. [Fig. 56] is a drawing of one seen in optical section in a whole mount of part of a proboscis, and shows a definite connection with the endoderm of the proboscis. This was the only case when such connection was satisfactorily established, but in sections it was not uncommon to find what seemed to be the remains of the broken stalk, as in [Fig. 54] (rs?). No connection could be traced between the cysts and any other part of the nervous system. As to function, the idea that they serve to give perception of space relations suggests itself as readily as any other hypothesis.

We come now to the consideration of the terminal knob of the clubs, the sensory portion proper. A complete and detailed account of the complex structure of these organs would fill many pages and involve much useless repetition. Claus (’78) has described them with accuracy, but not in great detail, and since then Schewiakoff (’89) has given a careful general description and has supplemented Claus’s work by observations upon the finer structure made with the aid of more recent technique. It seems in place for me, therefore, to give in the briefest possible way a general idea of their structure, and to pass then at once to the points in which my work has led me to different conclusions from those of Claus and Schewiakoff. In brief, then, the knob of the sensory club consists of a thick, complex mass of nerve fibres, more or less imbedded in which lie the special sensory organs, surrounding the ampulla-like terminal enlargement of the canal. The surface between the special organs is covered with less specialized sensory epithelium. The sensory organs are seven in number. Of these, four are simple invaginations of the surface epithelium arranged in two pairs symmetrically to the median line in the proximal end of the knob (the end where the stalk enters) and having pigment developed in the cells so invaginated, while the space of the invagination is filled with a gelatinous refracting secretion. These are considered simple eyes. Two more of the organs are complex eyes situated on the median line of the inner surface of the knob, the upper one smaller than the lower, but having almost exactly the same structure. Each has a cellular lens over which extends a superficial, corneal layer of cells; below the lens a refractive “vitreous body”; and below this a retina with pigmented cells. The seventh organ is the crystalline sac, which lies almost at the end of the knob opposite to the stalk and contains a large concretion. In view of the fact that the sensory clubs in toto have been abundantly figured by Claus and Schewiakoff, it is my intention to give but one simple figure of the general relations, and I justify that one in that it was made from the fresh material. [Fig. 57] is a camera sketch of the outlines given by a sensory club seen in optical section from the side. The smaller upper and the larger lower complex eyes which are situated on the mid-line, are seen in profile, while the two small simple eyes give the outlines that they would in a surface view of their side of the knob. Of course it is understood that two similar ones would appear on the other side, since the four simple eyes are symmetrically paired on either side of the mid-line. The sketch seems to show at least this much, that even in the living state the lens of the larger eye projects out beyond the other contours of the surface, so that the marked convexity ascribed to it in descriptions is not to be attributed to the preservation.