THE SIPHONOPHORA. 



127 



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

 always free and flexible, the ectoderm developing no hard 

 chitinous exoskeleton, save in the case of the pneumatophores 

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

 but in Velella and JPorpita, 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. 2i.~A(horybia rosacea.— A, lateral view ; B. from above; C, 2), detached hydro- 

 phyllia ; a, polypites ; 6, tentacles ; c, sacculi of the tentacles ; d, hydropbyilia ; 

 /, pneumatophoie. 



principal function of which is to develop the gonophores 

 from their pedicles. In these two genera the tentacula are 

 separate from the hydranths, and form the outermost circle 

 of appendages. 



The hydranths of the Siphonophora (Fig. 25, A) never 

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

 expanded, is trumpet-shaperl. The endoderm of the hydranth 

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



The aboral surface of the umbrella was of a brownish-srrav color, variesrated 

 with oval white spots ; the oral surface, liorht brown with eisrht bluish-green 

 lines radiating toward the lithocvsts ; the brachia, gray with brown dpts.^ 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 clavate tentacles being white. The long tentacles which 

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

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

 calwith the Cephea ocellata of Peron and Lesueur? The individual figured 

 was a young male. 



