THE SIPHONOPHORA. 



127 



3. The Siphonophora. — In this group the hydrosoma is 

 always free and flexible, the ectoderm developing no hard 

 chitiiious exoskeleton, save in the case of the pneumatophores 

 of some species. In most, the hydranths are of equal size ; 

 but in Velella and Porplta^ the hydranth situated in the 

 centre of the discoidal body is very much larger than the 

 rest, which occupy a circumferential zone around it ; and the 



Fig. 2L—Athoryhm rosacea.— A, lateral view ; B. from above; C, 7), detached hydro- 

 phyllia ; a, polypites ; 6, tentacles ; c, succuli of the teutacles ; c?, hydropbyllia ; 

 /, pneumatuphoie. 



principal function of which is to develop the gonophores 

 from their pedicles. In these two genera the tentacula are 

 separate from the hj^dranths, and form the outermost circle 

 of appendages. 



The hydranths of the Riplionopliora (Fig. 25, A^ never 

 possess a circlet of tentacula round the mouth, which, when 

 expanded, is tnmipet-shaped. The endoderm of the hydranth 

 is ciliated, and villus-like prominences project into its cavity. 



The aboral surface of the umhrella was of a brownish-^ray color, variesrated 

 ■nith oval white spots ; the oral surface, lis^ht brown with ei<rht bluish-green 

 lines radiatinir toward the lithocysts ; the brachia, srrav with brown dots. The 

 brachia divide into two at their origin, and then subdivide into an infinity of 

 small branches. The general color of the smaller branches is light brown, the 

 small interspersed clavatc tentacles being white. The long tentacles which 

 terminate each brachium are blue and cvlindrical at their origin, but become 

 trigonal ftirther on, where they are shaded with brown and green. Is it identi- 

 cal with the Cephea ocdlata of Perou and Lesueur? The individual figured 

 was a young male. 



