X THE VOYAGE OF H.M.S. CHALLENGER. 



medusas no rhopalia (tentacles instead). These differences are shown by the following 

 tables : — 



§ 10. Survey of the two sections of the Acraspedae. 



Tesseronle (Tetraperias). 



Acraspeda? without sense clubs, or with four sense 

 clubs (rhopalia). Umbrella usually highly arched, 

 conical ; four perradial gastral pouches large, sepa- 

 rated by developed cathammal septa. 



Order I. Stadeomedusje, Pis. X V.-XVII. 

 (System, p. 363, taf. xxi.-xxii.) 

 No rhopalia ; simple tentacles instead. 



Order II. Peromedus*:, Pis. XVIH-XXV. 

 (System, p. 396, taf. xxiii., xxiv.) 

 Four interradial rhopalia and four perradial tentacles 

 between (often eight adradial tentacles in addition). 



Order DX Cubomedus^, PI. XXVI. 

 (System, p. 423, taf. xxv., xxvi.) 

 Four perradial rhopalia and four interradial tentacles 

 or bunches of tentacles between. 



Ephyronle (Oetoperia;). 



Acraspedae with eight or more (up to thirty-two) sense 

 clubs (rhopalia). Umbrella usually depressed, 

 discoid ; four perradial pouches small, usually 

 become part of the central stomach, by dissolution 

 of the cathammal septa. 



Order IV. Discomedus;e, Pis. XXVII.-XXXH. 

 (System, p. 450, taf. xxvii.-xL) 

 Eight rhopalia or more, four perradial, and four 

 interradial (sometimes also several accessory). 



Sub-Order I. Cannostomae. 

 GSsophagus simple, without free oral arms, 

 tacles usually short and solid. 



Ten- 



Sub-Order II. Semostomas. 

 Oesophagus cleft into four large perradial, folded oral 

 arms. Tentacles usually long and hollow. 



Sub-Order III. Rhizostoma?. 

 (Esophagus represented by eight adradial dice-shaped 

 oral arms with numerous funnel openings. Central 

 mouth fused. No tentacles. 



§ 11. Polyphyletic origin of the Craspedotae. The section Craspedotaa or Hydro- 

 medusas is probably a polyphyletic animal group ; several different groups of Craspedotas 

 having arisen independently of one another from several different groups of Hydropolyps. 

 This is corroborated by the fact that all Anthomedusae are descended from Tubularian 

 Polyps, all Leptomedusse from Campanularian Polyps. The phyletic divergence of these 

 two groups of polyps is probably very much older than the origin of the two correspond- 

 ing orders of Medusae. The Trachylinas or Cordyliotae seem to form a third independent 

 group of this section, the Trachomedusas and Narcomedusae. But these two orders may 

 also have originated independently of each other. The monophyletic origin of all forms 

 within the four orders of Craspedotae from a single common ancestral form, though not 

 certain, is still extremely probable (System, p. 359). 



