The structure of the Cubomedusæ seems to be that of a type well established, and accordingly offers no very wide range of diversity among the different genera. The Charybdea that has just been described is a very typical form and will serve well as a standard with which to compare our species of Tripedalia. The resemblances are so close that a detailed account of the anatomy of the second form would involve much needless repetition. It is hardly necessary to do more than merely point out in what points Tripedalia resembles Charybdea and in what points it differs.
The form of the bell is less pyramidal than in Charybdea. Some measurements even gave the breadth greater than the height. The external surface is divided, as typical for the Cubomedusæ, into the four perradial sides and the four convex interradial ridges, and the furrows that separate these areas are with one small exception exactly the same as those of Charybdea, as may be seen by comparing the series of sections of Tripedalia ([Figs. 21-30]) with those of Charybdea ([Figs. 6-15]). The exception is almost too slight to mention. The adradial furrow in each octant which sets off the corner rib from the perradial surface in the lower part of the bell is not directly continuous, as in Charybdea, with the corresponding furrow in the upper part of the bell—that is, the afr´ of [Figs. 24-27] is not continuous with the afr of [Figs. 22 and 23], as is seen by both being shown in [Fig. 24]. The upper furrow (afr) is continued only a short distance, however, below the starting point of the lower (afr´).
The pedalia conform entirely to the description given those of Charybdea, except that there are three attached to the bell margin in each interradius instead of one, and that the blade of each pedalium is much narrower.
The sensory clubs also show exactly the same relation to the bell and exactly the same structure.
In the bell cavity the proboscis has a longer and better defined stalk than that of Charybdea, and has the further and more important difference of possessing special sensory organs, to the number of fifteen or twenty. The suspensoria are much more developed than in Charybdea, so that the interradial funnels lying between are more marked. In a corresponding way the frenula are larger and stouter ([Figs. 28, 29], frn). The musculature shows no new features and differs only in being comparatively more strongly developed and having a more pronounced striation. The nerve ring follows the same looped course from the margin in each interradius up to the level of the sensory clubs in the perradius.
c. Internal Anatomy.
The stomach offers no peculiarities, and the phacelli also agree with those of Charybdea except in having a smaller number of filaments in each tuft. The stomach pockets are not guarded by such well-developed valves as those described for Charybdea, though the valvular nature of the lips of the gastric ostia is indicated and the valvular functions undoubtedly performed. The gastric ostia are smaller (cf. Figs. [7] and [22]), and this makes highly developed valves less necessary. No trace of anything corresponding to mesogonial pockets was noticed.
In the matter of the marginal pockets, however, we find that the agreement with Charybdea is no longer continued. The regions that correspond to the eight marginal pockets of Charybdea are formed, as in that genus, by the coming together of the exumbrella and subumbrella at the sensory niche ([Figs. 25-28]), but each of these regions is subdivided, as it is not in Charybdea, into two marginal pockets, a larger (mp, [Figs. 28-29]) and a smaller (mp´). In this way sixteen marginal pockets are formed as in the Chirodropidæ. Furthermore, as happens in the latter family but does not in the Charybdeidæ, the marginal pockets extend into the velarium. From each of the larger marginal pockets are given off two velar canals, while each of the smaller gives rise to but one short one ([Fig. 18]). [Fig. 30] represents one of the last sections of a Tripedalia cut transversely, in which nothing but the pedalia and the velarium appear, and in it are shown the velar canals (vc), which come from the larger marginal pockets. The velarium appears in four segments because it is drawn upwards in the four perradii by the frenula (see [Fig. 20]). That the canals from the smaller pockets do not appear in the section is due to their shortness and to the fact that they are pulled upwards above the level of the sections by the frenula, together with that portion of the velarium.
The smaller velar canals, a pair in each perradius, seem to have in the males some function in connection with the storing of matured spermatozoa. In specimens with ripe testes they are very often found crowded to distension with spermatozoa, while the other velar canals may or may not contain them, and generally do not. The epithelium lining them is, like that of the others, composed of columnar cells higher on the wall turned toward the bell cavity than on that turned towards the exterior, but otherwise not specially differentiated. I searched in vain for any trace of opening by which the spermatozoa might gain the exterior. [Fig. 29] shows another point which may be mentioned in passing, namely, that the canal of each of the three tentacles opens into the peripheral gastro-vascular system independently. The central tentacle of each group is the homologue of the single tentacle of Charybdea, and is formed in Tripedalia before the two lateral tentacles appear. Its communication with the peripheral pocket system is higher up than the openings of the lateral tentacles, so that in the section drawn the latter are just beginning to be indicated (ct´).
It remains only to speak of the reproductive organs of Tripedalia. The sexes are separate in this form also, and ovaries and testes have the same structure as is found in other Cubomedusæ. The development of floating masses of cells in the females, however, is a feature which, so far as I know, has not been observed before. These masses, of which a small one is represented in section by [Fig. 71], are apparently developed along with the eggs, and repeat the structure of the ovary to all intents the same as if they were various-sized fragments of it broken loose. They consist mostly of high, columnar epithelial cells surrounding a few central cells and showing here and there a nettle cell just as the reproductive organ does. The epithelial cells differ from those of the ovary in containing one or more large vacuoles, and this vacuolation increases as the embryos, among which the masses float, develop. The idea naturally suggests itself, therefore, that they serve for nourishing and perhaps for protecting the embryos while they are developing in the stomach pockets of the mother individual.