220 
THE VOYAGE OF H.M.S. CHALLENGEE. 
bearing manifest oscules ; in which case the sponge would, of course, have to be removed 
from the genus Cydonium. 
The cortex is 2 ‘5 mm. thick, and is almost entirely constituted by the sterrastral 
layer. • The long spicules of the body pass through it to the exterior, but not in great 
numbers ; about their distal ends, however, as they pass through the cortex, a considerable 
number of additional long spicules make their appearance, their proximal ends lying 
in the outer half or two-thirds of the cortex, their distal ends lying several millimetres 
beyond its surface. The distal ends are generally broken off, but such as have been 
observed are simply pointed, so that most of these hispidating spicules are oxeas. Some, 
however, are modified into tylotoxeas, with large spherical tylotes, which lie embedded 
in the outer layer of the cortex, the development of the tylotes standing, no doubt, in 
direct relation to the tensions to which the embedded ends of such long projecting spicules 
are necessarily exposed. In the case of many Monaxonid sponges the tylote is developed 
in spicules similarly situated. 
The cladomes of the dichotrisenes before extrusion from the sponge lie between the 
outer epithelium and the sterrastral layer, ^.e., in the ectochrote, and this is reduced to 
such thinness that no room is left for any forwardly directed projection of the cladi, 
hence their extension at right angles to the rhabdome almost immediately on proceeding 
from their origin. Thus the marked depression of the cladome into a plane at right angles 
to the rhabdome is to be correlated with the reduction of the ectochrote. 
The choanosomal tissue is crowded with a profusion of foreign bodies — skeletons of 
Radiolaria, of various species of Foraminifera, spicules of Calcareous, Hexactinellid, and 
Monaxonid sponges, plates of Echinoderms, including the wheel-like scleres of Chiro- 
dota, form the bulk of them ; a few grains of sand also occur, and deciduous sterrasters 
of the sponge* itself. 
The structure of the sterraster is more than usually well displayed in this sponge. 
Looked at en face the ends of the actines are seen to extend laterally into from four to 
six recurved spines (PI. XXL fig. 37) ; when by a tangential section the spined ends are 
removed the actines are still seen as separate projecting pillars, but with a circular out- 
line, surrounding an area which is distinguished into a central peripheral portion by 
a slight difference in refractive index (PI. XXL fig. 39). Slightly deeper tangential 
sections through the solid mass of the spicule reveal the actines as still independent rods, 
with a circular outline and differentiated central portion, but between them and uniting 
them into a solid mass is seen a further deposit of silica, which can be distinguished from 
the actines by a slight, difference in refractive index (PI. XXL fig. 40). In a longitudinal 
section the actines are seen as conical rods having their apices at the centre of the spicule 
and their base outwards. Interstitial silica cements them together almost up to their 
termination ; where this ceases they extend as separate pillars with concave sides to end in 
the slightly expanded summit from which the recurved spines proceed (PI. XXL fig. 38). 
