30 GUIDE TO THE FOSSIL INVEETEBKATE ANIMALS. 
Gallery X, (collav-celLs), SO that they closely resemble certain Protozoa ; 
interspersed among tliese are other cells, each of whicli is 
pierced by one of the very minute pores mentioned above ; 
the substance surrounding the chambers is a jelly containing 
various cells, among them the germ-cells and the spicule- 
cells. For further information as to the soft parts, recourse 
should be had to the exhibit in the Zoological Department. 
All tills soft mass of the sponge is supported by a 
skeleton built up from the tiny spikes or rods deposited by 
the spicule-cells. In the common bath-sponge, as in many 
modern sponges, the substance of the skeleton is horny, and 
appears incapable of preservation in the fossil state. In 
Fig. 8. — Fossil sponge spicules : Silicispongiae, Heteractinellida, skeletal 
spicule (a) ; Silicispongiae, TetractineUida and others, flesh spicules 
(b-g) ; Calcispongiae, skeletal spicules (h-n). a is from Asteracti- 
nella ; b, c from Geodia; j-l from Tremacystia. a is enlarged 13 
diam. ; b-g, G6 diam. ; h, 26 diam. i, 114 diam. ; j, m, n, 134 diam. ; 
k, I, 80 diam. (After Hinde.) 
other sponges the skeleton is either calcareous, i.e. composed 
of carbonate of lime (calcite), or siliceous, i.e. imposed of 
flinty spicules. Each of these latter is further distinguished 
by being deposited around an axial filament of softer tissue, 
which disappears in the fossils, leaving an axial canal. The 
siliceous sponges constitute the larger and more important 
group, and are the better preserved as fossils ; in some of 
them ’ the siliceous skeleton partly gives place to horny 
fibres, a change which suggests how the true horny sponges 
£irOS6. 
The spicules, whether of calcite or of silica, are built 
on certain plans which are utilised in classification. The 
main types are: (1) Monaxons (Fig. 14), spicules of rod- 
like form, that grow out from a single point of origin either 
