270 COELENTERATA HYDROZOA chap. 



Dicoryne, which forms spreading colonies on Gasteropod shells 

 .and has free gonophores provided with two simple tentacles, 

 while the other organs of the medusome are remarkably degenerate. 

 In Garveia and Eudendrium the gonophores are adelocodonic, in 

 the former genus arising from the body-wall of the axial zooids 

 of the colony, and in the latter from the hydrorhiza. Stylactis is 

 sometimes epizoic (p. 268). Among the genera that are usually 

 placed in this family, of which the medusome stage only is 

 known, are Lizzia (a very common British Medusa) and Rathkea. 

 In Margelopsis the hydrosome stage consists of a single free- 

 swimming zooid which produces Medusae by gemmation. 



Fam. Podocorynidae. The zooids have the same general 

 .features as those of the Bougainvilliidae, but the perisarc does 

 not extend beyond the hydrorhiza. 



In Podocoryne and Hydractinia belonging to this family the 

 hydrorhiza forms an encrusting stolon which is usually found on 

 Gasteropod shells containing a living Hermit crab. In Podo- 

 coryne the gonophores are free-swimming Medusae with a short 

 manubrium provided with labial tentacles. Hydractinia differs 

 from Podocoryne in having polymorphic zooids and adelocodonic 

 gonophores. 



A fossil encrusting a Nassa shell from the Pliocene deposit of 

 Italy has been placed in the genus Hydractinia, and four species 

 of the same genus have been described from the Miocene and 

 Upper Greensand deposits of this country. 1 These are the only 

 fossils known at present that can be regarded as Gymnoblastic 

 Hydroids. 



The Medusa Thamnostylus, which has only two marginal ten- 

 tacles and four very long and profusely ramified labial tentacles, 

 is placed in this family. Its hydrosome stage is not known. 



Fam. Clavatellidae. This family contains the genus Clava- 

 tella, in which the zooids of the hydrosome have a single circlet 

 of capitate tentacles. The gonophore is a free Medusa provided 

 with six bifurcated capitate tentacles. 



Fam. Cladonemidae. This family contains the genus Clado- 

 nema, in which the zooids have two circlets of four tentacles, the 

 labial tentacles being capitate and the aboral filiform. The 

 gonophore is a free Medusa with eight tentacles, each provided 

 with a number of curious capitate tentacular processes (Fig. 131). 



1 Carter, Ann. Mag. Nat. Hist. (4) xix. 1877, p. 44 ; (5) i. 1878, p. 298. 



