Tripedalia endured captivity much more hardily than the Charybdea, and would live in aquaria happily enough for a number of days--no attempt was made to see how long. Specimens with their stomach pockets filled with ripe spermatozoa, or with young at any stage from egg to planula, were taken in plenty from the latter part of June to the latter part of July. In each female the young were all at the same stage. The embryos were thrown out in the aquaria as free-swimming planulæ, which settled down on the bottom and sides of the glass in a day or two, and quickly developed into small hydras with mouth and typically with four tentacles (and four tænioles, W. K. B.), though three and five were by no means uncommon. In this condition they lived for three weeks without essential change, and they were still giving no promise of further development when the laboratory broke up and the jars had to be emptied.
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´_).