PORIFERA 35 



In some groups, as for instance in the Monaxonida and 

 Tetractinellida, the spicules are not united or are joined 

 by spongin only ; but in others they are fused together or 

 interlocked so as to form a complete scaffolding, and 

 generally it is in these only that the external form of the 

 sponge has been preserved in the fossil state. In most 

 siliceous sponges, two kinds of spicules may be distin- 

 guished, the skeletal- spicules or megascleres which build 

 the main part of the skeleton, and the flesh-spicules or 

 microscleres which are smaller and isolated and are seldom 

 preserved as fossils. In the axis of each spicule there is 

 a canal known as the axial canal (fig. 9, c), which in the 

 living sponge is occupied by a thread of organic matter ; 

 this is the first part of the spicule to be formed, the mineral 

 matter being subsequently deposited around it. 



The spicules of recent siliceous sponges are characterised 

 by the glassy appearance of their surface, and by the silica 

 being colloidal, isotropic, and soluble in heated caustic 

 potash. But in the fossil state the spicules have generally 

 undergone considerable change ; occasionally their silica 

 is still colloidal but the surface has no longer the glassy 

 appearance, and the axial canal is frequently filled with 

 secondary silica in a crystalline or crypto-crystalline con- 

 dition, and is consequently easily distinguished by the aid 

 of polarised light when the spicule itself still remains 

 colloidal. Generally, however, the spicule has become 

 crystalline or crypto-crystalline, and in such cases the axial 

 canal can rarely be detected since it is filled with material 

 in the same condition. Sometimes the silica of the spicules 

 has been entirely removed, a hollow cast only remaining ; 

 in other cases it is replaced by another mineral, as for 

 instance by calcite in the sponges from the Lower Chalk 

 of Folkestone, by iron pyrites in Protospongia from the 



3—2 



