TRACHOMEDUSAE. 135 



Order 5. TRACHOMEDUSAE.* 



Hydromedusae without hydrosome (polyp stage); with marginal 

 sense-tentacles in pits or vesicles, ivith endodermal otoliths. Ocelli 

 usually absent. Gonads radial. Radial canals 4, 6, or 8, oft&jrwith 

 centripetal canals. With .thread-cell thickening of ectoderm round the 

 edge of the umbrella. 



The medusae of this order (Fig. 101) develop directly from the 

 egg, and no polyps are known. 



Fam. 1. Petasidae. With 4 radial canals and 4 gonads, stomach without 

 stalk, with sense-tentacles sometimes free, sometimes in vesicles. 



Sub-fam. 1. Petachnidae. Without centripetal canals. Petasus H-., 

 Dipetasus H. ; Petasata H. ; Petachnum H. ; Aglauropsis F. Miiller ; Gossea 

 L. Ag. 



Sub-fam. 2. Olindiadae. With blind centripetal canals. Olindias 

 F. Miiller. 



Fam. 2. Trachynemidae. 8 radial canals and 8 gonads, without stomach- 

 stalk, sense-tentacles rarely free, usually in vesicles. 



Sub-fam. 1. Marmanemidae. Tentacles without suckers, mesogonionsf 

 absent. Trachynema Gegenbaur ; Marmanema H. ; Hhopalonema Gegenbaur. 

 Sub-fam. 2. Pectyllidae. Tentacles with suckers, with 8 radial meso- 

 gonions.t Pectyllis H. ; Pectis H. ; Pectanthis H. 



Fam. 3. Aglauridae. With 8 radial canals, stomach with stalk, sense 

 tentacles free. 



Sub-fam. 1. Aglanthidae. With 8 radial gonads (sometimes 011 the 

 stomach-stalk, sometimes on the sub-umbrella). Aglantha H. ; Aglaura 

 Peron and Les. ; Agliscra H. 



Sub-fam. 2. Persidae. 4 or only 2 opposite gonads. Stauraglaura H. ; 

 Persa McCrady. 



Fam. 4. Geryonidae. With 4 or 6 radial canals and flattened gonads ; with 

 long stomach-stalk ; with 8 or 12 marginal umbrella-clasps or peroniums ; and 

 with 8 or 12 closed tentaculocysts (sense tentacles enclosed in vesicles), which are 

 embedded in the jelly on the axial side of the peroniums (Fig. 102). 



The tentacles are in three different groups which appear at three different 

 periods of the development. (1) The primary tentacles are transitory larval 

 organs, and filled with solid endoderm. They are perradial and 4 or 6 in 

 number. They pass on to the ex-umbrella, and remain connected with the 

 edge by a peronium. (2) The secondary tentacles are interradial and also solid, 

 and 4 or 6 in number. They pass on to the ex-umbrella, and remain connected 

 with the edge by peroniums. They may fall off or persist. (3) The tertiary 

 tentacles develop last and persist. .They arise beneath the perradial primary 

 tentacles just to one side of the peroniums of these latter. They are hollow 

 and long, and their cavity opens into the circular canal. There is a solid 



* E. Haeckel, Monographic der Medusen. Jena, 1879. 



f Mesogonions are thin vertical radial folds of the sub-umbrella, which underlie 

 the radial canals and divide the gonads into two separate halves. They incom- 



g'etely divide the umbrella cavity into spaces recalling the funnel-cavities of the 

 eriphyllidac. 



