Ixxxviii 
THE VOYAGE OF H.M.S. CHALLENGER. 
more crowded together towards the outer epithelium, are found immediately outside it 
embedded in foreign matter which encrusts the surface ; in Calthropella simplex a small 
spheraster occurs, and this is found scattered amongst the spicules of a small sponge, 
Astropeplus pulcher (p. 453), which incrusts it. 
In CinacTiyra harbata again the radical anatrisenes are extruded in such profusion 
that they remain entangled together, forming a dense basal mass which serves as a 
support; a similar case is met with in Thenea wyvillii; the most conclusive proof, how- 
ever, is furnished by Synops neptuni (p. 227), a sponge richly traversed by epoch ets, 
most of which are completely filled — “stuffed” would best express it — with deciduous 
spicules evidently derived from the sponge itself ; this will appear from the fact, first, 
that they are all of the same shape and dimensions as those constituting the skeleton 
of the sponge, no admixture occurring either of foreign spicules or of the sedimentary 
detritus of the sea floor ; and next, because there is no conceivable way by which these 
spicules could have been introduced into the epochets, except by extrusion from the 
sponge itself. The sponge is as much as 40 cm. high, and no one would think of 
invoking currents to introduce a pure gathering of spicules from the sea floor into 
labyrinthine cavities 30 to 40 cm. above it. 
General evidence might very well have led us to conclude that an extrication of 
spicules from the living sponge really occurs, but nothing less than direct observation of 
the actual process could have furnished such conclusive proof as that afforded by the pre- 
ceding instance, and even direct observation, unless long continued, would not have given 
us so clear an idea of the surprisingly large quantity of spicules which are thus cast out. 
That this process must have an important bearing on the question of the origin of 
flints is obvious, and some remarks under this head will be found on p. 280. 
Source of the Silica in Flints. — Since sponges have furnished some, and indeed no 
inconsiderable portion of the silica of flints, it becomes of interest to determine in a few 
instances the quantity of silica present in a sponge as compared with its total bulk, and 
hence if possible to frame an estimate of the time that would be required for the forma- 
tion of a bed of flints. 
The first step is to determine the total bulk of the sponge, including all contained 
cavities ; various methods were devised to accomplish this, all giving more or less con- 
cordant results, but the following was found to be the simplest and least inaccurate : — A 
spirit specimen was transferred from alcohol to distilled water which was repeatedly 
changed till all traces of alcohol were removed. It was then totally immersed by means 
of a wire cage in well-boiled water contained in a weighing bottle, and the bottle and 
its contents were weighed ; the sponge was then removed in its wire cage, the cage 
returned to the bottle, and a second weighing gave by difference the weight of the sponge 
together with that of the water which filled its interstices and coated its surface ; in the 
case of densely hirsute sponges, the results would from capillarity have been too high, 
such sponges were therefore somewhat differently treated ; after removal from a beaker of 
