80 THE VOYAGE OF H.M.S. CHALLENGER. 



Stylactella spongicola, n. sp. (PI. II. figs. 5, 6). 



Habitat. — Northern and Central Pacific, symbiotic with Stannomidee, Spongelidse, 

 and Psamminidas; Stations 241, 244, 270, 271, 272, 274; depths between 2300 and 

 2900 fathoms. 



Stylactis with a reticular hydrorhiza, the anastomosing tubes of which are 

 cylindrical and of equal breadth. Hydranths ovate or clavate, springing at short 

 intervals from the hydrorhiza, sessile or very shortly pedunculate, with a single 

 circlet of eight (?) tentacles. Gonophores ovate, twice as large as the hydranths, arising 

 scattered between them from the hydrorhiza. 



Stylactella spongicola is by far the most frequent among the Eydroids which live 

 in symbiosis with the Deep-sea Keratosa. It is the usual symbiotic Hydropolyp in 

 all the species of Stannomidse, and occurs too in some Spongelidse (Psammophyllum, 

 Cerelasma) and in some Psamminida? (Psammopemma). Its hydrorhiza traverses the 

 body of these sponges in all directions, and replaces the absent strong spongin-fibres, 

 giving to the sponge a firm support and a distinct form. Since the network of the 

 hydrorhiza is continuous throughout the whole sponge, it reaches in the largest species 

 of Stannophyllum (venosum) the enormous size of 1 00 to 200 mm. and more. The 

 polygonal or roundish meshes of the network are of variable diameter, usually between 

 1 and 0"5 mm., but sometimes they are far larger, 3 to 5 mm. or more, at other times 

 only - 2 to - 4 mm. The chitinous tubes of the network are cylindrical, of nearly equal 

 breadth, usually between - 05 and 0"1 mm., but sometimes 0'12 mm. and more 

 (PI. II. fig. 6, h). The thin, yellowish, chitinous wall is of variable thickness. The 

 entoderm of the hydrorhiza, inside the chitinous tubes, is always dark coloured, greenish 

 brown or yellowish brown. 



The hydranths are usually developed in the superficial layers of the network only, 

 and mainly in the distal portion of the sponge. But in many specimens they are 

 difficult to observe, and sometimes I failed to discover them at all. They are most 

 easily examined in those Stannomidae in which the skeleton is for the most part 

 composed of Globigerina ooze. After having dissolved the calcareous matter by hydro- 

 chloric acid, there remains the transparent and colourless mesoderm of the sponge, in 

 which the dark network of the hydrorhiza is easily seen (PI. II. fig. 5). 



The hydranths of Stylactella spongicola are ovate or club-shaped, and spring at 

 irregular intervals directly from the branches of the hydrorhiza. They are very small, 

 only 0"2 to - 3 mm. in diameter, whilst the hydranths of Stylactis vermicola (Allman, 

 loc. cit.) are ten times as large. The small basal pedicle, which is very distinct in 

 this latter, invested with a chitinous perisarc ("rudimental stem," Allman), is wanting 

 in the former species. The small claviform or oviform hydranths exhibit at the distal 



