30 GUIDE TO THE FOSSIL INVEETEBEATE ANIMALS. 



Gallery X. flagellated chambers bear each a lash surrounded by a collar 

 (collar-cells), so that they closely resemble certain Protozoa ; 

 interspersed among these are other cells, each of which 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 this 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, Tetractinellida 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, 66 diam. ; h, 26 diam. ; 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. composed of 

 flinty spicules. The latter are 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 

 arose. 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 



