230 
THE YOYAGE OF H.M.S. CHALLENGER. 
cavities stands in no insignificant ratio to the mass of its skeleton. In tfie figures (PI. 
XXV.; PI. XXVI. fig. 1) some of tlie cavities still filled with spicules are indicated, 
but many of those represented as empty were filled when the sponge was cut open, and 
afterwards cleared out to better understand the nature of the cavity. In a paper 
dealing with the formation of flints, I suggested (while ignorant of the deciduous nature 
of sponge spicules) the following analogy : ^ — 
“ Excepting that sponges do not periodically shed their spicules like leaves and spores, 
the explanation we have just suggested bears a striking resemblance to the ‘ growth in 
place ’ theory of our coal beds. In the coal, as in the flints, the structure of the con- 
stituents has generally been almost entirely obliterated, yet some few of the leaves or 
spicules, as the case may be, are occasionally found in an admirable state of preservation ; 
and just as a Sigillaria every now and again remains a solitary survivor of a whole forest, 
so now and then a whole sponge is to be found preserved out of a host of associates now 
vanished or turned into flint.” 
It would now seem (leaving out the word “periodically”) that the exception with 
which I guarded this analogy disappears, and the source of the silica of flints becomes 
less perplexing. At the quiet bottom of a chalk sea the spicules as they were shed would 
remain strewn around the parent sponge, and undergoing solution from the moment they 
were liberated, would commence the transformation of chalk into flint 'pari p>cissu with 
the life of the sponge. 
The cortex of the sponge is about 0'8 mm. thick (PL XXVI. figs. 11-13). It consists 
almost entirely of the sterrastral layer, covered externally by a layer of subepithelial 
chiasters. Small oxeas project from its outer surface in the neighbourhood of the pores, 
and the cladomes of the orthotrisenes extend within the inner fibrous layer. The oscules 
are from 0'08 to 2'0 mm. in diameter. Although the sponge is a dried specimen, sections 
were cut from it in the usual way ; the cribriform roof of the poral chones and the 
sphincters of the oscules were thus demonstrated (PL XXVI. figs. 11-13); it is a 
remarkable fact that the fibrous structure of the tissue binding together the sterrasters 
of the cortex and the fibrous structure of the inner layer of the cortex are still preserved ; 
and not only so, but the fibres of the muscular sphincters, characteristically difierent 
from those of the inner cortex, and sometimes possessing a well-preserved nucleus, are 
distinguishable. Even indications of flagellated chambers are occasionally to be made 
out in the choanosome, and a vesicular character in the collenchyma, which occurs in 
the neighbourhood of the cortical layer of the sinuses of the wall. This latter layer 
is very thin, the sterrasters are accumulated to form a stratum 0'16 mm. thick, and 
beneath this follows a somewhat close accumulation of oxeate spicules, lying chiefly 
parallel with the surface, many with much enlarged canals. It appears as though these 
were dead spicules, on the way to be thrust out to the exterior. 
^ Sollas, On the Flint Nodules of the Trimmingham Chalk, Ann. and. Mag. Nat. Hist, ser. 5, vol. vi. p. 443, 1880. 
