164 THE INVERTEBRATA 



processes which engulf decaying polyps, epizoic organisms like 

 diatoms and protozoa and larvae of other epizoic forms. 



Sertiilaria (Fig. 122 B) with a creeping hydrorhiza, more or less 

 branching stems which bear opposite hydrothecae; hydrothecae 

 large, so that the polyps can completely retract within them. 



The following genera of Gymnoblastea may also be mentioned : 



Cordylophora, living in fresh or brackish water (Norfolk Broads), 

 polyps with scattered filiform tentacles. 



Pennaria (Fig. 122C) with two kinds of tentacles, oral capitate 

 and aboral filiform; nematocysts of very large size; medusae de- 

 generate but become free when gonads are mature. 



Hydr actinia^ with spreading plate-like perisarc covered by naked 

 coenosarc, very often found coating a shell inhabited by a hermit 

 crab; with spiral dactylozooids and sessile gonophores. 



Podocoryne, as Hydr actinia, but with free medusae. 



The polyp forms of many medusae, both Antho- and Lepto- 

 medusae, are unknown. 



Order TRACHYLINA 

 This group consists of forms in which the medusoid develops directly 

 from the egg and the polyp has either been reduced to a minute 

 fixed individual or is represented only by the planula larva which 

 metamorphoses into a medusa. The possession of sense tentacles with 

 endodermal concretions is an important character. There are two 

 suborders : 



Trachomedusae. Trachylina with sense tentacles in pits or vesicles 

 and with gonads situated in the radial canals ; with marginal tentacles 

 on the edge of the umbrella. Examples: Geryonia, Limnocodium, 

 Carmarina (Fig. 120 II), Limnocnida. 



Narcomedusae. Trachylina with sense tentacles not enclosed and 

 marginal tentacles inserted some distance aborally from the edge of 

 the umbrella ; with gonads on the oral wall of the stomach. Example : 

 Cunina. 



The inclusion of the following freshwater forms in the order is 

 provisional : 



Limnocnida is a remarkable freshwater form found in the Central 

 African lakes. Up till the present only male medusae have been found 

 in Lake Tanganyika and female in Victoria Nyanza. Asexual repro- 

 duction by budding takes place from the margin of the bell. Other 

 species occur in Rhodesia and the Indian rivers. 



Craspedacuta (Limnocodium) w^s first known from the Victoria Regia 

 tank in the Royal Botanic Gardens at Kew, but has now been dis- 

 covered in various North American rivers and has even colonized 



