6 
THE VOYAGE OF H.M.S. CHALLENGEE. 
tent-like eminences. Towards the base of the sponge the somal anatrisenes pass into the 
radical form (6), and the protrisenes become more numerous. The differentiation in form 
is evidently connected with a difference in function either in kind or more probably in 
degree. The rooting anatrisenes can only resist a pull, they have no rigidity capable of 
meeting a thrust, and hence we may regard them as strictly comparable in their action 
to the anchors used for securing ships at sea. 
The trichodal protrisenes are most abundantly developed around the edges of the pore- 
areas (PL I. fig. 12), and about the margin of the oscule. The number of the cladi is 
frequently reduced to two, or one, or they may be altogether absent, the distal club-like 
thickening from which they arise when present still, however, persisting. 
Owing to the tenderness of the ectosome, and the loose way in which the spicules of 
the fibres are aggregated, the sponge is extremely soft and easily torn. The course of 
the fibres is but slightly spiral. 
Tetilla pedif era, ^ Sollas (PI. XLI. figs. 6-21). 
Tetilla pedifera, Sollas, Prelim. Account, Sci. Proc. Eoy. Dubl. Soc., vol. v. p. 180, 1886. 
Sponge somewhat thumb-shaped, erect, sessile (?). Surface finely hispid, owing to 
the projection of brushes of protrisenes ; raised into irregular, somewhat tent-like 
eminences, which tend to form ridges ; oscules numerous, small, 1 mm. in diameter and 
less, usually situated on the summit of the tent-like projections ; oscular margin a wide 
smooth membrane, roofing over an excurrent canal of greater diameter than the oscule. 
Pores apparently distributed irregularly over the surface. Ectosome thin, supported 
by numerous oxeas, which lie in all directions, parallel to its surface. 
Spicules. — I. Megascleres. 1. Oxea, fusiform, anisoactinate, maximum thickness 
nearer the distal than the proximal end, the latter frequently attenuated to a filiform 
extremity, always more sharply pointed than the distal end, 3’213 by 0'03 mm. 
2. Protrisene (PL XLI. figs. 9-11). A slender conical rhabdome attenuated to a 
sharply pointed filiform extremity ; usually slightly swollen just below the origin of the 
cladi ; cladi simple, three, two, or only one in number, slightly curved, directed forwards, 
and only very slightly outwards, usually of unequal length ; when all three are present two 
are of about equal length, but much shorter than the third ; this form by the loss of one 
of the shorter cladi produces the variety with two, and by the loss of both the shorter 
cladi the variety with one cladus; rhabdome 2 ’38 by O'OllS mm.; the cladi vary slightly 
in length according to the number present; in the true trisene form, the longer cladus 
is about 0T5 mm., the shorter cladi about 0‘06 mm. long; in the disene, the longer is 
0T52, the shorter 0‘071 mm. long ; when only one cladus is present it measures 0T58 mm. 
in length. 
1 Pedum, a shepherd’s crook. 
