2 
THE VOYAGE OE H.M.S. CHALLENGER. 
less gradually to a point at tlie distal than at the proximal end, which frequently be- 
comes filiform; 2'326 mm. in length by 0‘0237 mm. in diameter. 
2. Trichodal oxea; immeasurably thin fusiform oxea, 0'395 mm. long, which, lying 
side by side with overlapping ends, give rise to spicular fibres (PI. I. fig. 27). 
3. Protrisene (PL I. figs. 21, 23-25). Rhabdome scarcely perceptibly less in diameter 
immediately below the cladome than near the actinal centre, thence tapering very 
gradually to a filiform termination. Cladi usually of unequal length, one, measuring 
0‘197 by 0'005 mm., longer than the other two, which are generally equal in size, viz., 
0‘0513 by 0‘0039 mm.; the cladi are also sometimes reduced in number, one or even 
both of the shorter disappearing, the longer cladus alone remaining then measures 
0’21 by 0'0054 mm. 
II. Microsclere. 4. Sigmaspire (PI. I. fig. 22). This is of the typical form, but is 
somewhat larger than usual, attaining a length of from 0’0237 to 0’0276 mm. 
Anatrisenes are absent. 
Colour. — Dark ashen-grey. 
Habitat. — Station 78, Azores, July 10, 1873; lat. 37° 26' N., long. 25° 13' W.; 
depth, 1000 fathoms; bottom, volcanic mud. 
RemarJcs. — Of this species, one of the simplest forms of Tetilla, two specimens 
were obtained; they are approximately of the same size, about 20 to 22 mm. long 
by 10 mm. wide. One bears a remarkable resemblance to a shoe or slipper, hence 
the name. The single oscule, 4 '7 mm. wide, occupies a position corresponding to the 
opening of the shoe ; its margin is produced into a short tube, supported by spicules, 
including numerous protrisenes, which lie within its walls parallel to its length ; 
internally it leads into a cloacal chamber, and into this the excurrent canals open freely 
by unconstricted apertures. In the other more fusiform specimen, the oscule is more 
nearly terminal in position, but otherwise similar. The opposite end of the sponge, 
corresponding to the toe of the shoe, is produced into numerous short papillae, from 
which pro trisenes, but no anatrisenes, project for a short distance outwards. These no 
doubt are reduced anchoring filaments. The sponge appears to have rested on one side, 
corresponding to the sole of the shoe, and if so the anchoring filaments would be without 
anchoring function, which might account for their reduction. 
The spicules diverge spirally from an excentric node in short fibres which project 
obliquely beyond the surface of the sponge, rendering it hispid ; the longer cladi of the 
protrisenes usually have a common direction over limited areas. 
The spicular fibres are of two kinds, those formed of large oxeas and protrisenes, and 
those which consist solely of trichodal oxeas : at the oscular margins, the spicular fibres 
enter the wall of the oscular tube and assume a close palisade arrangement. 
The exterior of the sponge (PI. I. fig. 27) is covered by a dermis (ectoderm and 
