294 COELENTERATA — HYDROZOA chap. 



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.'- 



The character of the manubrium and the position of the 

 sexual cells suggest that Limnocnida has affinities with the 

 N'arcomedusae or Anthomedusae, but the marginal sense-organs 

 and the number and position of the tentacles, showing consider- 

 able similarity with those of Limnocodium, justify the more con- 

 venient 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 'f. 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 manubriiim there is a 

 rod-like projection of the mesogloea from the aboral wall of the 

 gastric cavity, covered by a thin epithelium of endoderrn, 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. 



^ Cf. Boulenger, Presidential Address to Section D of the British Association 

 (Cape Town, 1905). 



