lxxviii 



THE VOYAGE OF H.M.S. CHALLENGER. 



Discomedusse, the rich group of the Ehizostomse (System, 1879, pp. 464, 560, taf. 

 xxxiv.-xl.) is distinguished by an extremely peculiar development of the mouth, unique 

 of its kind in the whole animal kingdom. This has arisen phylogenetically from the 

 second sub-order, the Semostomse (loc. cit., taf. xxx.-xxxiii.), as the latter is derived from 

 the Cannostomse, loc. cit., taf. xxvii.-xxix.). This plainly points out the ontogeny of the 

 Ehizostomse, which are Cannostomse in their earliest stage, then become Semostomae, 

 and are finally transformed into Ehizostomse. The simple quadrangular oesophagus of 

 the Cannostoinse first shows four delicate, frilled oral lobes at the oral margin (Ephyridse, 



Fig. K. Cannorhiza connexa (Discomedusse, Versuridre). 



Adradial section, (ug) Gelatinous umbreUa. (gc) Central stomach, (gg) Bottom of the central stomach 

 (gastrogenital membrane with the genitalia, s). (ir) Subgenital porticus. (ah) Brachiferous plate. 

 (ap) Arm Pillars, (cd) Pillar canals, (ga) Buccal stomach, (ab) Oral arms (adradial). (cb) 

 Branchial canals, (am) Funnel frills (sucking mouths). 



System, taf. xxvii. ; Flosculidse, taf. xxvii.). As these increase considerably in size, and 

 the intermediate four interradial oral sinuses take the form of deep incisions, they are 

 transformed into four strong, perradial oral arms, which are developed in most Ulmaridse 

 into long oral pennons (System, taf. xxxi.-xxxiii). In one Ulmarid (Aurosa, System, 

 taf. xxxiii. figs. 7, 8), the four perradial oral arms are bifurcated at the distal end into 

 two lobes. In this way, the eight adradial oral arms are formed, which are present in 

 all Ehizostomse, and which are only connected in pairs at their bases ; where they are 

 usually connected with four strong perradial oral pillars (ap)- Whilst the thin leafdike 



