Fam. 1. Charybdeidae.—Cubomedusae with four interradial tentacles.

Charybdea appears to have a very wide geographical distribution. Some of the species are usually found in deep water and come to the surface only occasionally, but others (C. xaymacana) are only found at the surface of shallow water near the shore. The genus can be easily recognised by the four-sided prismatic shape of the bell and the oral flattened expansion of the base of the tentacles. The bell varies from 2-6 cm. in length (or height) in C. marsupialis, but a giant form, C. grandis,[[354]] has recently been discovered off Paumotu Island which is as much as 23 cm. in height. The colour is usually yellow or brown, but C. grandis is white and C. xaymacana perfectly transparent.

"Charybdea is a strong and active swimmer, and presents a very beautiful appearance in its movements through the water; the quick, vigorous pulsations contrasting sharply with the sluggish contractions seen in most Scyphomedusae." It appears to be a voracious feeder. "Some of the specimens taken contained in the stomach small fish, so disproportionately large in comparison with the stomach that they lay coiled up, head overlapping tail."[[355]]

Very little is known of the development, but it is possible that Tamoya punctata, which lacks gonads, phacellae, and canals in the velum, may be a young form of a species of Charybdea.

Fam. 2. Chirodropidae.—Cubomedusae with four interradial groups of tentacles.

This family is represented by the genera Chirodropus from the Atlantic and Chiropsalmus from the Indian Ocean and the coast of North Carolina.

Fam. 3. Tripedaliidae.—Cubomedusae with four interradial groups of three tentacles.

The single genus and species Tripedalia cystophora has only been found in shallow water off the coast of Jamaica. Specimens of this species were kept for some time by Conant in an aquarium, and produced a number of free-swimming planulae which settled on the glass, and quickly developed into small hydras with a mouth and four tentacles. The further development of this sedentary stage is unfortunately not known.

Order II. Stauromedusae.

This order contains several genera provided with an aboral stalk which usually terminates in a sucker, by means of which the animal is temporarily fixed to some foreign object. There can be little doubt that this sedentary habit is recently acquired, and the wide range of the characteristic features of the order may be accounted for as a series of adaptations to the change from a free-swimming to a sedentary habit.