164 
THE VOYAGE OF H.M.S. CHALLENGER. 
open pores in the epithelium ; though it forms the roof to numerous dome-like canals 
extending up to it. It is, of course, quite possible that pores may open in this epithelial 
roof when the sponge is actively alive, and that they close up when it is exposed to 
unfavourable conditions, such as immersion in alcohol. However this may be (and I think 
it unlikely) the chief supply of water to the sponge evidently flows through the incurrent 
tube, which delivers it to the intercortical cavities or superficial branches of the incurrent 
canals ; thence it descends by the usual incurrent canals into the choanosome, and is 
urged on by the choanocytes through the flagellated chambers into the excurrent canals, 
whence it flows into the cloacal tube, and by pores in the walls of this reaches the 
exterior. 
The Cortex . — The exterior of the whole sponge is invested with an epithelial layer, 
over the under face of which are scattered innumerable sanidasters ; this then forms the 
outermost layer of the cortex. It is succeeded by collenchyma, which presents the usual 
stellate collencytes, but with the branching processes strongly developed, so as to produce 
a general fibrous reticulation. In addition numerous fusiform cells are present, most, 
but not all, of which are gathered into fibrous tracts, which wrap round the spicules 
of the cortex, binding them together into a strong framework. Next the choanosome 
the fibres are more richly developed than elsewhere, forming a darkly staining layer. 
Immediately beneath the epithelium is a layer of collencytes more or less fusiform in shape, 
their long axes lying radially, with short processes extending from their distal ends at 
right angles to the epithelium, with which they come in contact, and longer processes 
from their proximal ends descending into the collenchyma. The cortex is densely 
crowded with oxeas (No. 2), which lie chiefly horizontally in aU azimuths ; it also 
receives the ends of the radiating sheaves of the chief skeleton (PI. XVIII. fig. 14). 
These include both oxeas and cladoxeas, the latter chiefly orthomonsenes (3, 7), their 
single cladi projecting into the cortex away from the side of the sheaf, in the same 
manner as the corresponding spicules do in Chrotella macellata, among the Tetillidse 
(see p. 20). 
Numerous irregular canals traverse the cortex, apparently in all directions, without 
any tendency to a chonal arrangement. According to universal rule they are lined by 
epithelium, which is accompanied by the usual sanidasters. The cortex is about 0*0714 
to 1*0 mm. thick. 
The Choanosome . — The mesoderm of the choanosome is a sarcenchyma, richly provided 
with fusiform cells, which are arranged about the spicular sheaves, both longitudinally 
and transversely, the transverse fibres enveloping the spicules and binding them together. 
Fibres cross from one spicular sheaf to another, concentrically surround the water canals, 
and wander apparently aimlessly among the flagellated chambers, which are frequently 
circumscribed in groups by fibrous strands. The flagellated chambers (PI. XVIIL figs. 
16, 22) measure 0*0276 mm. in breadth, by 0*0237 mm. in length, the apopyle is about 
