REPORT ON THE TETRACTINELLIDA. 
67 
the walls of the incurrent canal, which starts from the poriferous recess, is a simple 
form of Hydroid, lying with its tentacles just below the surface of the canal and its 
basal end extending inwards amongst the flagellated chambers. It is about 0'3 mm. 
long by 0'05 mm. broad at the oral end. The cavity of the collenchyma in which it 
lies is lined by the epithelium of the sponge and opens by a somewhat constricted 
aperture into the canal. I have more than once found Hydroids overgrowing the 
surface of a sponge, but never before met with one inhabiting the interior. 
Thenea schmidtii, SoUas (PL VII. flgs. 1, 2 ; PI. VIII. flgs. 21, 22). 
Stelletta agariciformis, O. Schmidt, 1870, Spong. Atlant. Gebiet., p. 68, pi. vi. lig. 12. 
Tisiplionia agariciformis, 0. Schmidt, Spong. Meerh. Mexico, 1880, p. 71. 
Thenea schmidtii, SoUas, Prelim. Account, Sci. Proc. Roy. Dubl. Soc., vol. v. p. 183, 1886. 
Sponge (PI. VII. flg. 1) more or less spherical in form ; special poriferous area either 
restricted to one side, opposite the oscule, which is then also lateral, or surrounding the 
sponge as an equatorial recess, which divides an upper portion, bearing one or more 
oscules, from a lower, which is produced into anchoring filaments. Surface hispid, 
margins of poriferous and oscular areas fringed by projecting spicules. Collenchyma but 
scantily developed in the choanosome ; flagellated chambers large. 
Spicules. — I. Megascleres. 1. Oxea, fusiform, sharply or bluntly pointed, straight or 
curved, 14 '3 by 0‘079 mm. The hispidating oxeas of the oscular margin and the 
tegminal edge are usually more slender (from 0‘045 to 0'06 mm. thick) than those of the 
body-fibres, from which the foregoing measurements are taken. In a specimen with a 
lateral poriferous area I found the distal ends of these spicules preserved in several 
instances.; and they proved to be not oxeas but protrisenes, the cladi of which are 
liable to reduction, sometimes all but one disappearing, in others all, a club-like thickening 
(tylus) then replacing them. 
2. Protrisene I rhabdome conical, straight or flexuous, with straight conical cladi 
diverging outwards and forwards. Ehabdome 5'4 by 0'071 mm.; cladi 1'07 by 
0’063 mm. 
3. Dichotrisene ; rhabdome straight or irregularly flexuous, sharply or bluntly 
pointed ; protocladi diverging outwards and forwards, deuterocladi tangential. Ehabdome 
•9‘3 by 0‘087 mm.; protocladi 0T75 mm. long; deuterocladi 1'5 by 0'055 mm. 
4. Somal anatrisene; rhabdome cylindrical, with a sharply pointed or rounded 
end ; actines diverging at first almost at right angles, subsequently somewhat suddenly 
bent backwards, till nearly parallel with the rhabdome. Ehabdome 17 ”9 by 0‘02 mm.; 
cladi 0T43 by 0'008 mm.; chord 0T2 mm. 
5. Radical anatrisene ; rhabdome as in the preceding ; cladi curving backwards 
and outwards from their origin, thicker, and more divergent than those of the somal 
