REPORT ON THE DEEP-SEA KERATOSA. 



79 



identical with that of Stylactis vermicola, but not with that of the three species of true 

 Stylactis which originally constituted this genus. I am therefore of the opinion that it 

 would be better, and justified by general systematic principles, to retain the older defini- 

 tion given by Allman in 1871 (p. 302), viz. : — "Gonosome: Sj)orosarc borne on the 

 hydranths at the proximal side of the tentacles," and to separate the symbiotic deep-sea 

 species as a new genus, Stylactella, " with gonophores borne on the creeping stolon or 

 the hydrorhiza." The full definition of this genus would then be as follows : — 



Stylactella, nov. gen. — Tubularise without hydrocaulus, with a reticular hydrorhiza, 

 from which arise single sessile or pedunculate hydranths, and scattered between them 

 single gonophores. Hydranths claviform, naked, with a single circlet of filiform tentacles, 

 which surround the base of a conical hypostome. Gonophores ovate, naked, with a 

 simple central spadix. Chitinous perisarc investing only the tubular branches of the 

 hydrorhiza, ' 



Species of Stylactella. 



1. Stylactella vermicola, Allman, Report, loc. cit., p. 2, pi. i. fig. 2. — Symbiotic with 

 an Annelid. Station 244 ; depth, 2900 fathoms. 



2. Stylactella spongicola, n. sp. — Symbiotic with many Deep-sea Keratosa (Spongelidse 

 and Stannomidse). Stations 241, 244, 270 to 274, &c. ; depths between 2000 and 2900 

 fathoms. 



3. Stylactella abyssicola, n. sp. — Symbiotic with several Deep-sea Keratosa 

 (Spongelidse and Stannomidse). Stations 198, 270 to 272, &c. 



The genus Hyclranthea, Hincks, is also similar to our Stylactella. Allman, in his 

 Tubularian Monograph (p. 301), places it between Wrightia (Atractylis) and Stylactis. 

 Comparing the figures of his Hyclranthea margarica, which Hincks gave in 1863, 1 

 I find it rather different, not only in the formation of the hydranth (with a short hydro- 

 caulus and a double circlet of tentacles), but also in the formation of the gonophores. 

 These are true medusiform sporosacs, with four radial canals in the rudimentary umbrella. 

 The gonophores of Stylactella, however, like those of Stylactis, are simple club-shaped 

 sacs, with a central blind-canal or spadix, between which and the ectodermal membrane 

 the ova are developed. I am much inclined to regard this formation as a primitive one, 

 not as having arisen from reduced Medusoids (as in the case of Hydranthea). I suppose 

 that Stylactella (and probably also Stylactis and some allied genera) belong to the 

 oldest and most primitive forms of Hyclroids, and that their gonophores are not reduced 

 Medusoids, but either simple genital buds, organs of the hydranth (as in Hydra), or 

 sexual zooids, separated from the nutritive zooids by division of labour. Perhaps 

 Stylactella and the allied genera may represent together a distinct family, Stylactidse. 



1 Ann. and Mag. Nat. Hist., ser. 3, vol. x. pi. ix. fig. 4. 



