The principal families of the Trachomedusae are:—
Fam. Olindiidae.—This family appears to be structurally and in development most closely related to the Leptomedusae, and is indeed regarded by Goto[[326]] as closely related to the Eucopidae in that order. They have two sets of tentacles, velar and exumbrellar; the statocysts are numerous, two on each side of the exumbrellar tentacles. Radial canals four or six. Manubrium well developed and quadrate, with distinct lips. There is an adhesive disc on each exumbrellar tentacle.
Genera: Olindias, Olindioides, Gonionema (Fig. 139), and Halicalyx.
As in other families of Medusae the distribution of the genera is very wide. Olindias mülleri occurs in the Mediterranean, Olindioides formosa off the coast of Japan, Gonionema murbachii is found in abundance in the eel pond at Wood's Holl, United States of America, and Halicalyx off Florida.
Two genera may be referred to in this place, although their systematic position in relation to each other and to other Medusae has not been satisfactorily determined.
Fig. 139.—Gonionema murbachii. Adult Medusa, shown inverted, and clinging to the bottom. Nat. size. (After Perkins.)
Limnocodium sowerbyi is a small Medusa that was first discovered in the Victoria regia tanks in the Botanic Gardens, Regent's Park, London, in the year 1880. It has lately made its appearance in the Victoria regia tank in the Parc de la Bête d'Or at Lyons.[[327]] As it was, at the time of its discovery, the only fresh-water jelly-fish known, it excited considerable interest, and this interest was not diminished when the peculiarities of its structure were described by Lankester and others. It has a rather flattened umbrella, with entire margin and numerous marginal tentacles, the manubrium is long, quadrate, and has four distinct lips. There are four radial canals, and the male gonads (all the specimens discovered were of the male sex) are sac-like bodies on the sub-umbrellar aspect of the middle points of the four radial canals. In these characters the genus shows general affinities with the Olindiidae. The difficult question of the origin of the statoliths from the primary germ layers of the embryo and some other points in the minute anatomy of the Medusa have suggested the view that Limnocodium is not properly placed in any of the other orders. Goto,[[328]] however, in a recent paper, confirms the view of the affinities of Limnocodium with the Olindiidae.
The life-history of Limnocodium is not known, but a curious Hydroid form attached to Pontederia roots was found in the same tank as the Medusae, and this in all probability represents the hydrosome stage of its development. The Medusae are formed apparently by a process of transverse fission of the Hydroid stock[[329]] similar in some respects to that observed in the production of certain Acraspedote Medusae. This is quite unlike the asexual mode of formation of Medusae in any other Craspedote form. The structure of this hydrosome is, moreover, very different to that of any other Hydroid, and consequently the relations of the genus with the Trachomedusae cannot be regarded as very close.
Limnocodium has only been found in the somewhat artificial conditions of the tanks in botanical gardens, and its native locality is not known, but its association with the Victoria regia water-lily seems to indicate that its home is in tropical South America.