TRACHYMEDUS.E — LIMNOCNIDA. 



371 



entoderm sharing in their formation. After this asexual process has ceased, gonads are 

 developed in the ectoderm of the stomach-wall. When set free the budded medusa has only 

 8 short, stiff tentacles, but medusa-buds of another generation begin to develop upon the 

 sides of its manubrium even before the new medusa is set free. The medusa pulsates before 

 being set free and while still attached to the side of the stomach of the parent. Sometimes 

 the medusa-buds, instead of being produced individually, are borne upon branched stolons, 

 which arise from the sides of the stomach. These stolons may break off and their buds 

 cause them to swim about, reminding one of Siphonophorae. Very soon, however, the pul- 

 sating medusae become detached and swim away separately. The budding process is often 

 so vigorous that the stomach-wall may be nearly destroyed by it. 



The gonads and tentacles are white to milky, all other parts being of a glassy transparency. 



This medusa is exceedingly abundant in the fresh-water Lake Tanganyika, Central 



Africa, from May to October, being commonest in June and July. Sexually mature medusae 



are found from May to October, but asexually reproducing 

 medusae are found throughout the year (Gunther). The medusa 

 is rare during the wet season, from November to April. The 

 natives call them the "eyes of the lake," and say that the lake 

 sleeps during the wet season and therefore the eyes are not 

 commonly to be found at that time. 



Gravier and Moore have found this or a very similar medusa 

 in Lake Victoria Nyanza. It is possible that both Lakes Victoria 

 Nyanza and Tanganyika constituted parts of the Jurassic Sea 

 of Central Africa, or possibly the medusa may have been intro- 

 duced into Victoria Nyanza by commerce from Tanganyika. 



Apparently a similar medusa is found at Sotuba, Niger 

 River, Africa, in lake-like expansions of the river above the rapids 

 (see du Guerne, 1893). Browne, 1906, also records a medusa with 

 more sense-organs and tentacles than those of Lake Tanganyika, 

 in the Forcados River, a fresh-water branch of the Niger, 102 

 miles from the sea, and in 1908 he describes it from Assay, Delta 

 of the Niger. 



Gunther, 1907, finds that there appear to be only male 

 medusae in Lake Tanganyika, whereas in Lake Victoria Nyanza all of the medusa; are 

 females. The medusa of Lake Victoria Nyanza appears to be a subspecies of that of Tan- 

 ganyika, and Gunther, 1907, calls it L. tanganicce war. victor ice. It is about 13 mm. in diameter 

 when adult and the older tentacles are deeply sunken into the gelatinous substance of the 

 bell. Moreover, only the outer two-thirds of the older tentacles are covered with nematocyst- 

 warts. 



Gunther, 1907, states that the medusae of Lake Tanganyika have normally 4 radial- 

 canals, but that the number ranges from 4 to 7 and that 24 per cent have 5 or more canals. 

 The supernumerary canals are not caused by a forking of the original 4. The free medusa 

 2 mm. wide has tentacles of the fifth order just beginning to appear and there are then only 

 16 lithocyst-vesicles. When the medusa is adult and 22 mm. wide the tentacles of the 

 eighth order and a few of the ninth are present. 



Fig. 211. — Limtwcnida tanganjicce, 

 after Moore, in Tanganyica 

 Problem. 



Family PTYCHOGASTRID.E. 



Pectyllidx, Haf.ckel, 1879, Syst. der Medusen, p. 265. 



FAMILY CHARACTERS. 



Trachymedusae with marginal tentacles grouped into more or less well-defined clusters. 

 Some of the tentacles bear adhesive disks. There are numerous free sensory-clubs. 8 radial- 

 canals. 4 lips. Stomach 8-lobed, with the lobes in the radii of the 8 radial-canals. These 

 stomach-lobes are bound to the subumbrella by means of 8 mesenterial partitions. The 

 gonads are upon the sides of these^8 stomach-lobes and each gonad is divided, more or less, 

 by the mesentery, so that there may be 8 double (16) gonads. 



