20 
THE VOYAGE OF H.M.S. CHALLENGEE. 
in diameter; tlie rhabdome 6‘5 byO‘0158 mm., is nearly cylindrical for a considerable 
distance below tbe cladome, and ends in a fibform termination. 
II. Microscleres. 8. Sigmaspire of the choanosome. Tbis is of tbe usual form; 
0'012 to 0'016 mm. long. 
9. Toxospire of the cortex (PI. IV. figs 11, 12). Tbis is a sigmaspire modified by 
continued growth in a spiral direction; it bas tbe form of a widely open belix of nearly 
two turns wntb a variable spiral angle. Its length is from 0’03 to 0'04 mm. 
Colour. — Dark grey. 
Habitat. — Station 208, off Manila, January 17, 1875; lat. 11° 37' N., long. 123° 31' E.; 
depth, 18 fathoms; bottom, blue mud. 
Remarks. — Four specimens of tbis sponge were trawled; all more or less hemi- 
spherical or dome-shaped with a flat base. Tbe largest measures 42 mm. by 38 mm. 
along its horizontal diameters, and 32 mm. in height. 
Tbe oscules (PI. IV. fig. 2) are borne on tbe summits of more or less conical 
eminences on tbe upper surface of the sponge ; tbe smallest specimen bears two, tbe 
largest six, having an average diameter of somewhat under 2 mm. They lead into a large 
cloacal chamber, tbe wall of which is perforated by numerous small round or oval apertures, 
giving it a cribriform appearance. Tricbodal protrisenes lie in bundles in tbe waU of tbe 
chamber and project obfiquely forwards, surrounding tbe pores with their cladomes. 
Skeleton. — The long spicules (1, 3, 4, 5, 7) are collected into radiating fibres, the 
trisenes appearing at tbe distal ends. From tbe terminal sheaf of tbe fibre tbe protrisenes 
with widely diverging cladi (4, 5) project their cladi into tbe cortex (PI. IV. fig. 18), thus 
contributing essentially to its support. Tbe cladomes of many of tbe anatrisenes, indeed 
almost aU, are also embedded in tbe cortex. Tbe highly porrectate protrisenes (3) project 
beyond tbe surface as bispidating spicules. Tbe numerous smaller oxeas (2) lie 
irregularly scattered through tbe cortex, crossing one another in directions oblique to tbe 
general surface, but more nearly parallel than perpendicular to it. Around tbe oscule 
the spicular fibres project in a kind of fringe; these fringing spicules are chiefly 
protrisenes. Tbe sigmaspires are scattered generally through tbe choanosome, but are 
most abundant in tbe walls of tbe canals and cloacal chambers. 
Tbe toxospires of the cortex are most abundant beneath tbe external epithelium, where 
they form a dense layer. Outside the sponge, covering tbe epithelium, is a quantity of 
granular debris, in which numerous deciduous toxospires occur. The crowding of the 
toxospires towards the exterior, and the presence of deciduous examples beyond it, point 
to an outward movement and final extrication of these spicules. 
Ectosome. — Beneath the external epithelium the cortex consists of a layer of 
collenchyma, very difficult to analyse, partly on account of its complexity, partly by 
reason of the slight but numerous variations which it exhibits in difierent regions. The 
