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.
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 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.
Fam. Trachynemidae.—In this family there are eight radial canals, and the statorhabs are sunk into a marginal vesicle. Trachynema, characterised by its very long manubrium, is a not uncommon Medusa of the Mediterranean and the eastern Atlantic Ocean. Many of the species are small, but T. funerarium has sometimes a disc two inches in diameter. Homoconema and Pentachogon have numerous very short tentacles.
Fam. Pectyllidae.—This family contains a few deep-sea species with characters similar to those of the preceding family, but the tentacles are provided with terminal suckers. Pectyllis is found in the Atlantic Ocean at depths of over 1000 fathoms.
Fam. Aglauridae.—The radial canals are eight in number and the statorhabs are usually free. In the manubrium there is a rod-like projection of the mesogloea from the aboral wall of the gastric cavity, covered by a thin epithelium of endoderm, which occupies a considerable portion of the lumen of the manubrium. This organ may be called the tongue. Aglaura has an octagonal umbrella, and a manubrium which does not project beyond the velum. It occurs in the Atlantic Ocean and Mediterranean Sea.
Fam. Geryoniidae.—In this family there are four or six radial canals, the statorhabs are sunk in the mesogloea, and a tongue is present in the manubrium. Liriope (Fig. 137) is sometimes as much as three inches in diameter. It has a very long manubrium, and the tongue sometimes projects beyond the mouth. There are four very long radial tentacles. It is found in the Atlantic Ocean, the Mediterranean Sea, and the Pacific and Indian Oceans. Geryonia has a wider geographical distribution than Liriope, and is sometimes four inches in diameter. It differs from Liriope in having six, or a multiple of six, radial canals. Carmarina of the Mediterranean and other seas becomes larger even than Geryonia, from which it differs in the arrangement of the centripetal canals.