The Cubomedusæ

by Franklin Story Conant

(g) Nerve Ring. It is in the possession of a clearly defined nerve ring that the Cubomedusæ differ from all other Scyphomedusæ whose nervous system has been carefully studied. The nerve ring shows very plainly on the surface of the subumbrella as a well-defined clear streak. Its course is zig-zag or festoon-like. In the interradii, at the basis of the tentacles, it lies not far from the bell margin. In the perradii it rises to the level of the sensory clubs. This very striking arrangement is understood at once when it is remembered that the sensory clubs represent the four perradial primary tentacles, and were originally situated on the margin. When all the rest of the margin grew down and away from the four sensory clubs, fusing below them to form the present intact edge of the bell, the four portions of the nerve ring that lay in the perradii were left at the level of the sensory clubs, and the originally straight nerve ring was thus bent into a bow in each quadrant. The finer structure of the nerve will be treated of in the special part to be devoted to the nervous system.

c. Internal Anatomy.

6. Stomach. The shape of the stomach is approximately that of a biconvex lens, as seen in [Fig. 4], which represents a Charybdea cut in halves longitudinally in the perradius. The lumen of the proboscis (the buccal stomach according to Haeckel’s terminology) communicates directly by a funnel-shaped enlargement with the stomach proper, or central stomach of Haeckel. The term basal stomach is carried over by Haeckel from the Stauromedusæ, where it has considerable significance, to the Cubomedusæ, and applied to the upper part of the central stomach. In the stalkless Cubomedusæ, however, it has no significance so far as actual structure goes, and our knowledge of the development of the Cubomedusæ is as yet too simple for us to say that the upper part of the main stomach represents what remains of the basal stomach of an earlier pedunculated stage.

The epithelium of the roof of the stomach is not specially differentiated and apparently has little or no part in digestion. The epithelium of the floor, on the other hand, is composed chiefly of very high and thickly crowded columnar cells which are usually described as coarsely granular, but under high powers appear to be filled with vacuoles surrounded by a network of cell substance. Thickly interspersed among these columnar cells are goblet cells filled with mucus. The floor is thrown into numerous wrinkles by ridges in the supporting gelatine resulting in increase of digestive surface. The four perradial grooves of the proboscis are continued in the perradii along the floor of the stomach as four fairly deep furrows, which lead directly to the gastric ostia and stomach pockets—structures to be described presently. These furrows are lined with crowded columnar cells, smaller and denser than the other cells of the digestive epithelium, containing no granules and but little beside the relatively large, compact, deeply staining nuclei. The furrows probably represent special ciliated courses.

7. Phacelli. Lying in the four interradial corners of the stomach are the four phacelli or tufts of gastral filaments to the number of thirty or thirty-five in each tuft. The filaments are attached to a single stalk, like the fringe of an epaulette or the hairs of a coarse brush. The stalk bearing the filaments is an outgrowth of the lower wall of the stomach just at the point where it fuses with the upper. The phacelli are therefore structures of the subumbrella, proof of which will be found under the special topic of the vascular lamellæ. The stalk, an indication of which appears in sph. [Fig. 6] (the section being a little below the axis of the stalk, which lies horizontally), consists of a firm core of gelatine covered with the high columnar epithelium of the floor of the stomach. The filaments themselves are slender processes repeating the structure of the stalk and having a central axis of gelatine for support covered with glandular epithelium, which in the case of the filaments bears numerous nettle cells. These processes are extremely contractile, and in the living animal show a continuous, slow, squirming movement like a mass of worms. The section just referred to ([Fig. 6]) shows diagrammatically three of these filaments (fph) cut across in each quadrant.

8. Peripheral Part of the Gastro-vascular System. The proboscis and stomach proper comprise the central part of the gastro-vascular system. In direct communication with the central is a peripheral part composed of pouches or pockets lying in the vertical sides of the cube-shaped bell, just as the central stomach lies in its roof. The peripheral part may be subdivided for convenience of description into the stomach pockets, the marginal pockets, and the canals of the tentacles and sensory clubs.

(a) Stomach Pockets. These are four broad, thin pouches lying between the exumbrella and the subumbrella in the four perradii (e. g. [Fig. 9], sp) and separated from one another in the interradii merely by four thin vertical strips of vascular lamella (ivl) or fusion between the two endodermal surfaces of a primitively single undivided peripheral cavity. The structure is exactly that which we should have if in a Hydromedusa, for example Liriope (Trachomedusæ), the four radial canals broadened out and the intervening cathammal plates correspondingly narrowed, until the relations in size were just reversed, and instead of four narrow radial canals separated from one another by four broad cathammal plates, we had four broad radial canals or pouches separated by four narrow cathammal plates.

The stomach pockets communicate at their top with the central stomach by means of four moderately large openings, the gastric ostia. These are seen in a side view of the whole animal as triangular spaces ([Fig. 1], g. o.) near the top of the broad perradial sides. In [Figures 7 and 8] they are seen in cross-sections, in [Fig. 4] in vertical section.

The communication between the stomach and each stomach pocket is guarded by a valve that can cut the one entirely off from the other. The valve is simply the flexible lower margin of the gastric ostium, a thin vertical fold of the floor of the stomach, semilunar in shape, just at the point where it is passing over into the stomach pocket. A longitudinal section, such as is shown in [Fig. 4], gives the best idea of the form and position of the valve that can be obtained from any simple section. Internal to the valve is seen a depression of the stomach wall, almost worthy to be called a pocket. The valve itself lies as a wall across the end of this depression, obstructing a free course to the stomach pocket. It will be seen at once that any pressure of fluids in the stomach against this vertical wall, or valve, would serve only to press it against the inner surface of the exumbrella, and thus effectually close the entrance into the stomach pocket. Such a closure would both keep the juices of the stomach from entering the pockets and the embryos in the pockets from entering the stomach before the proper time.

The depression of the floor of the stomach just internal to the valve may possibly be a structure of some morphological significance. In one series of sections it was found that in two of the quadrants the depression was deeper than that represented in [Fig. 4], and extended perceptibly into the outer or vertical portion of the suspensorium. [Fig. 32] is a diagram giving a vertical reconstruction in the perradius of the cross-sections in which this deepened depression was noticed. [Fig. 31] is a drawing (the outline by camera lucida) of one of the cross-sections, through the lowermost point of the depression. The figure gives the wall of the stomach lined with high columnar epithelium (ens), and the wall of the stomach pockets, with the suspensorium (su) connecting them. The section is taken just above the broad angle that lies between the two parts of the suspensorium, that is, in a plane parallel to the arrow a-b in [Fig. 32], but a little lower down. At the points to which the reference letter x ([Fig. 31]) refers are seen the first indications of the division into two parts, i. e. of the apex of the angle. The next section or two lower down show the relation seen in [Fig. 10] (su). There can be no doubt in this case that the depression or pocket lies in the outer vertical limb of the suspensorium. It is the position that gives it at least the appearance of some morphological significance. In two genera of Lucernaridæ named and described by Clark (’78), Halicyathus and Craterolophus, the mesogonia or suspensoria in all four perradii contain broad pockets. These mesogonial pockets in the Lucernaridæ have given rise to considerable misunderstanding owing to the fact that in some forms the reproductive organs bulge out from the stomach pockets in which they structurally lie, and come to take up a secondary position in the walls of the mesogonial pockets. The sections of Charybdea above referred to indicate that among the Cubomedusæ we may have the same structure reduced to its lowest terms, and may be a feather’s weight in favor of the view that the Cubomedusæ are descendants of an attached Lucernaria-like form.