DR. WYYILLE THOMSON ON HOLTENIA. 
709 
2. The wall of the oscular cavity . — There can be, I think, no reasonable doubt that 
the inhalation of water takes place through the pores of the ultimate sarcocle network 
over the whole of the external surface of the sponge, and that the whole of the wall of 
the internal cylindrical cavity is exhalent, the current finally escaping through the single 
large osculum. The wall of the inner cavity resembles generally the external wall of 
the sponge. It is composed of a series of stars formed on the same plan, and by a com- 
bination of spicules of the same general form as those of the external stellate membrane. 
The stars are coarser and more open towards the bottom of the cavity, becoming more 
obscure above, till at length near the oscular opening the tissue becomes close, irregular, 
and felted, and at the tip of the osculum it joins the upper portion of the outer wall which 
has assumed the same character. Under the microscope the network of the oscular cavity 
(Plate LXIX. figs. 4, 5) differs materially in detail of structure from the outer network. 
The spicules of the supporting frame are rather smaller, but their distribution is the 
same, the four secondary rays of each spicule forming an elastic union with the secondary 
rays of the neighbouring spicules to form the stars, and the fifth ray passing down into 
trabeculae of sponge-substance, which, however, are smaller than on the outer surface 
and less definite in form. The most striking peculiarity in the inner wall is the total 
absence of the ultimate sarcode network. The siliceous rays are bound together by 
a thick sheathing of firm sarcode, much more consistent and more loaded with granules 
than the sarcode of the outer wall ; but this sarcode merely covers the spicules and 
reduces the angular meshes of the network to large irregular oval spaces, by forming 
deep crescentic accumulations in the angles. Close ranges of the transverse crosses of 
the long delicate feathered spicules (Plate LXYIII. fig. 10) are imbedded in the sarcode 
and set against the shafts of the large spicules, and the feathered rays turn towards, and 
project slightly over, the oval openings, which they thus partially obstruct. The amphi- 
disci are very abundant, imbedded in the sarcode. They are usually more or less regu- 
larly ranged in rows, their long axes parallel with the axes of the large spicules. 
3. The Sponge-substance . — Between the outer and the inner walls the structure is very 
uniform. A greyish consistent sarcode is hollowed out into rounded channels and com- 
municating spaces of all sizes and of very irregular forms (Plate LXIX. fig. 1, Plate LXX. 
figs. 1 & 2). These spaces are smaller, and the sponge is therefore more compact towards 
the inner (oscular) surface, where it has somewhat the appearance in section of a slice 
of very light loaf-bread. About a centimeter from the outer surface the various channels 
coalesce and form large sinuses, which surround the columns of sponge already men- 
tioned which support the centres of the large stars. The effect of this arrangement is 
very peculiar ; when the sponge is seen in fluid, the outer network from its almost per- 
fect transparency is entirely lost, and one sees only a globular spongy mass, apparently 
covered with projecting spongy, broad-based, flat-headed spikes. Towards the oscular 
network there is also a tendency to the coalescence of the channels, but the sinuses 
formed are much smaller and less definite in form. The sarcode of the substance of 
the sponge is firmer and more densely loaded with granules, therefore more opaque, than 
mdccclxix. 5 B 
