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. _Limnocnida tanganyicae_ is another remarkable fresh-water Medusa, about seven-eights of an inch in diameter, found in the lakes Tanganyika and Victoria Nyanza of Central Africa.[330] It differs from _Limnocodium_ in having a short collar-like manubrium with a large round mouth two-thirds the diameter of the umbrella, and in several other not unimportant particulars. It produces in May and June a large number of Medusa-buds by gemmation on the manubrium, and in August and September the sexual organs are formed in the same situation. [Illustration: FIG. 140.—_Limnocnida tanganyicae._ × 2. (After Günther.)] The fixed hydrosome stage, if such a stage occurs in the life-history, has not been discovered; but Mr. Moore[331] believes that {294}the development is direct from ciliated planulae to the Medusae. The occurrence of _Limnocnida_ in Lake Tanganyika is supposed by the same authority to afford a strong support to the view that this lake represents the remnants of a sea which in Jurassic times spread over part of the African continent. This theory has, however, been adversely criticised from several sides.[332] The character of the manubrium and the position of the sexual cells suggest that _Limnocnida_ has affinities with the Narcomedusae or Anthomedusae, but the marginal sense-organs and the number and position of the tentacles, showing considerable similarity with those of _Limnocodium_, justify the more convenient plan of placing the two genera in the same family. FAM. PETASIDAE.—The genus _Petasus_ is a small Medusa with four radial canals, four gonads, four tentacles, and four free marginal statorhabs. A few other genera associated with _Petasus_ show simple characters as regards the canals and the marginal organs, but as very little is known of any of the genera the family may be regarded as provisional only. _Petasus_ is found in the Mediterranean and off the Canaries.