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THE VOYAGE OF H.M.S. CHALLENGER. 



not hydrocaulus. The entoderm of the coenosarc, enclosed in the chitinous cylindrical 

 tubes of the perisarc, is of the same dark colour as in Stylactis. A few gonangia were 

 observed scattered between the hydrothecse and arising immediately from the perisarcal 

 network (fig. 9, h). They had the same size and subovate form as those of Halisi- 

 phoriia megalotheca, but they were not compressed or spathuliform, with a slit-shaped 

 opening ; their transverse section and the distal opening are circular (fig. 9, g). 



The genera Stylactis and Stylactella. 



The genus Stylactis was founded in 1864 by Allman, in his leading paper on con- 

 struction and limitation of genera among the Hydroids. 1 It is the first genus of the 

 family Podocorynidse, and one of the most primitive among the Tubularise or Gymno- 

 blastic Hydroids. Allman's definition of Stylactis is as follows : — " Trophosome : Coenosarc 

 mainly composed of a retiform hydrorhiza, which consists of anastomosing tubes invested 

 by a periderm ; hydrocaulus rudimental or absent. Polypites claviform, with a single 

 verticil of filiform tentacles surrounding the base of a conical metastome. Gonosome : 

 Gonophores adelocodonic, borne on the body of the polypite at the proximal side of the 

 tentacles." Two species are mentioned, Stylactis fucicola ( = Podocoryne fucicola, Sars) 

 and Stylactis sarsii, Allman ( = Podocoryne carnea, Sars). 



Similar to this first definition of Stylactis by Allman is the one which he gave in 

 1871, in his excellent Monograph of the Gymnoblastic or Tubularian Hydroids, 2 where 

 he describes a third species, Stylactis inermis (p. 306). But afterwards, in his Challenger 

 Report, 3 the definition of Stylactis was essentially altered, and given in the following 

 words : — " Trophosome : Hydrocaulus rudimental, being reduced to short tubular processes, 

 which spring at intervals from a creeping, stolon-like hydrorhiza and support the hydranths 

 on their summit ; hydrorhiza destitute of external ccenosarcal investment. Hydranths 

 clavate, with a single circlet of filiform tentacles, which surround the base of a conical 

 hypostome. Gonosome: Gonophores adelocodonic, borne by the hydranth at the 

 proximal side of the tentacles, or by the creeping stolon." The new deep-sea species, 

 which Allman describes and figures, Stylactis vermicola? was found symbiotic on the back 

 of an Aphrodite-like Annelid, Lsetmonice producta, taken in the North Pacific at 

 Station 244, depth 2900 fathoms. 



This new deep-sea species, Stylactis vermicola, symbiotic with an Annelid, is of 

 particular interest, since it occurs at the same Station (Station 244, depth 2900 

 fathoms) as our Stylactis abyssicola, symbiotic with different Keratosa (Spongelidse 

 and Stannomidse). Considering the formation of the gonophores, which spring in this 

 latter directly from the hydrorhiza, and not from the body of the hydranths, I find it 



1 Ann. and Mag. Nat. Hist., ser. 3, vol. xiii. p. 353. 2 Allman, Monogr. of the Tubularian Hydroids, 1871, p. 302. 

 3 Zool. Chall. Exp., pt. lxx. p. 1. 4 hoc. cil, p. 2, pi. i. fig. 2. 



