Two more diagrams, [Figs. 33 and 34], are added in order to give a more complete understanding of a gastric ostium and its neighboring structures, the mesogonial pocket and the valve. [Fig. 33] is a view of the gastric ostium and valve from the stomach side, and represents the appearance that would be given by a thick section through the arrow x-y in [Fig. 32], in a plane at right angles to the paper. The heavy lines outlining the gastric ostium (enr and enfl) represent the place where the plane of the section has cut across the epithelium of the roof of the stomach above the ostium and the epithelium of the floor of the pocket-like depression internal to the valve. The continuation of the two heavy lines in either side of the ostium represents the region where the roof and floor of the stomach meet; i. e., the edge of the lens-shaped stomach. The semilunar outline of the valve (vg) is shown by a light line just above the epithelium of the depression. As is seen by the reference arrow in [Fig. 32], the valve lies a little external to the immediate plane of the section, and hence it is that its inner surface is seen in [Fig. 33] and not a section of it. The vertical part of the suspensorium (su) is seen in section below the epithelium of the depression. The reference numbers 1, 2, 3 and 4 denote the same points in [Figs. 32 and 33]. [Fig. 32] referred to [Fig. 33] would lie in a plane at right angles to the paper through the reference arrow x-v of the latter figure.
[Fig. 34] represents a horizontal section through the gastric ostium at the level of the arrow a-b in [Fig. 32], or arrow c-d in [Fig. 33]. The reference numbers 5, 6 and 7, 8 denote similar points in the two figures [33 and 34]. [Fig. 32] as referred to [Fig. 34] is through the arrow e-f; [Fig. 33] is through the arrow c-d. In the series of cross-sections, [Fig. 9] is taken at a level a little below that of [Fig. 34], and passes through the basal part of the valve (vg).
(b) Marginal Pockets. The part of the peripheral portion of the gastro-vascular system in each quadrant which is called the stomach pocket extends downwards as far as the sensory niche. Here by the coming together of the walls of the exumbrella and subumbrella the space between them is obliterated ([Fig. 15]) in the immediate perradius. From the sensory niche downward to the margin each stomach pocket is thus divided into two smaller pouches, the marginal pockets (mp). In each side of the Cubomedusan cube there are, then, in Charybdea two marginal pockets; or in all eight, a characteristic of the family Charybdeidæ. The marginal pockets as the name implies extend downwards to the bell margin, and are continued into the velarium as the velar canals. Of these ([Fig. 3]) there are two from each marginal pocket, or sixteen in all. The constancy in their number is one of the characteristics that distinguish C. Xaymacana from the very closely related C. marsupialis of the Mediterranean. (Compare [Fig. 3] with the similar one by Claus for C. marsupialis, ’78, Taf. I., Fig. 6.) The forked shape, while to be sure the common form in C. marsupialis, is an almost invariable characteristic in C. Xaymacana. It may be mentioned again that the presence of these canals is one of the chief features that distinguish the velarium of the Scyphomedusæ from the velum of the Hydromedusæ.
(c) Canals of the Sensory Clubs and Tentacles. The four interradial definitive tentacles and the four perradial transformed tentacles, the sensory clubs, are hollow, and their canals communicate directly with the peripheral part of the gastro-vascular system. The canal of the sensory club in each quadrant leads directly out from the stomach by a somewhat funnel-shaped opening formed by the approximation of the two walls of the stomach pocket. The relation of the canal of the sensory club to the stomach pocket is seen at a glance in [Fig. 37]. It is given by means of cross-sections in [Figs. 12-14]. [Figure 12] shows the inner walls of the stomach pocket approaching the outer at two points, leaving between them a concavity freely open to the rest of the stomach pocket above and at the sides. [Fig. 13], a little lower down, shows the two walls fused together at two points, making the interspaces a definite canal communicating with the stomach pocket above only. This canal lies directly over the sensory niche, and in the next figure ([No. 14]) the canal is seen to have passed through the roof of the sensory niche and to have entered the base of the stalk of the sensory club. In the enlarged end of the club, the part which bears the sensory structure, the canal widens into a terminal ampulla-like sac.
The endoderm lining the canal of the sensory club is specially differentiated. In the stalk it is more columnar than the epithelium of the stomach pockets, and is made up of cells containing a brightly staining nucleus with very little trace of cytoplasm. The cell bodies appear as if filled with a clear, non-staining fluid. Perhaps these cells give the stalk elasticity to act in connection with the thin layer of longitudinal muscle-fibres that are found just external to the supporting lamella. The epithelium of the terminal enlargement of the canal is composed of very high narrow cells, many of which show two nuclei of equal size and staining quality lying side by side.
In continuation of the specialized epithelium of the perradial furrows in the floor of the stomach the inner wall of the stomach pocket shows a strip of similar densely crowded columnar cells leading from the gastric ostium downwards to the canal of the sensory club. As in the other case, the strip probably represents a specially ciliated tract, and perhaps in it we see the reason why the canal of the sensory club is almost always found to contain either spermatozoa which are shed by the male reproductive organs directly into the stomach pocket, or else floating cells of the kind to be described in the next section.
The canals of the interradial tentacles arise from the peripheral gastro-vascular system much lower down than those of the sensory clubs, since these tentacles have preserved their primary positions with reference to the bell margin. [Figure 16] represents a section taken at the level of the base of the pedalia which gives the connection of the tentacle canals with the gastro-vascular system. At the level below the sensory niche the four broad stomach pockets have been divided, as we have seen, into the right marginal pockets (mp). The figure shows that in the interradial corners the longitudinal septa (ivl, in the preceding figures), or lines of fusion between the two walls of the peripheral gastro-vascular space, which divide the primitively simple space into the four stomach pockets, have come to an end, leaving a connecting canal (cc) in each corner as all that remains of the primitive uninterrupted communication between all parts of the peripheral system. It is from these four connecting canals that the tentacle canals take their origin. From this point of origin each tentacle canal passes downwards, surrounded by the gelatine of the pedalium, into the tentacle proper.
The connecting canals are of morphological importance in that they are supposed, with much reason, to represent in the Cubomedusæ the circular canal of the Hydromedusæ.
9. Reproductive Organs. The sexes are separate in Charybdea. In both sexes the reproductive organs consist of four pairs of long leaf-like bodies, each leaf attached along one edge to the wall of the subumbrella in an interradius (see [Fig. 1], r), and hanging free in the stomach pockets. From this position in the stomach pockets it is evident that the reproductive organs are endodermal. The lines of attachment of each pair is just internal to the longitudinal vascular lamella that fuses the outer and inner walls of the stomach pockets together in the interradius (ivl), and the reproductive organs are therefore structures belonging to the subumbrella. It is interesting to note how careful examination of the medusan organization takes away from the importance of the outer cup, the exumbrella, and adds to that of the inner, the subumbrella. We have seen that the phacelli and the sensory clubs, from whose position it would be supposed that they belonged to the exumbrella, are organs of the subumbrella, and that there is no muscle-tissue in the exumbrella; we find now that the reproductive organs belong to the subumbrella, and it will be shown later that the tentacles, like the sensory clubs, are structures of the subumbrella also. To the exumbrella are left only the functions of support and covering.
The mature reproductive organs extend very nearly throughout the entire vertical length of the bell, and are therefore found in the series of cross-sections in all but the uppermost and lowermost ([Figs. 7-15] r). The organs consist of germ cells within, covered by an epithelium of columnar cells that shows here and there nettle cells. The ova are found with different amounts of yolk, according to age, surrounding a large nucleus almost devoid of chromatin and an intensely staining nucleolus. In young ova there appears very plainly in every case at least one small deeply staining body inside the nucleus, which very much resembles the nucleolus. These are probably so-called yolk nuclei, and while I have not made a special study of the ovogenesis, I infer that the constant presence of at least one, points to an origin of the ovum from a syncytium (of at any rate two cells), similar to that which has been recently shown by Doflein (’96) to occur in the formation of eggs in Tubularia. In the nearly mature ovary each ovum is surrounded by a layer of gelatine, which comes from the gelatinous sheet that enters the leaf-like ovary for its support along its line of attachment just internally to the interradial septum. It seems as if the ova, arising in the epithelium on the surface, pushed their way into the gelatine inside and there completed their development entirely surrounded by a slight investment of gelatine, which grows thinner around each ovum as it increases in size. In males the testes always show a similar division into compartments by gelatinous meshes, the compartments thus mapped out being filled with the small brightly staining spermatocytes. Ova and spermatozoa when mature are set free in the stomach pockets.