28 PORIFERA. 



horny substance known as spongin, or of needles (termed 

 spicules} of various forms (fig. 6), composed of carbonate 

 of lime or of silica, or it may consist of both siliceous 

 spicules and spongin. It is only those forms which have 

 either a siliceous or calcareous skeleton that are definitely 

 known as fossils. In some groups the spicules are not 

 united, as for instance the Monactinellidse and Tetracti- 

 nellidse, but in others they are fused together or inter- 

 locked so as to form a complete scaffolding, and generally 

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

 can be preserved in the fossil state. In most sponges, 

 two kinds of spicules may be distinguished, the skeletal- 

 spicules which go to build the main part of the skeleton, 

 and the flesh-spicules which are smaller and isolated and 

 are seldom preserved fossil. In the centre of each spicule, 

 there is a tube known as the axial canal (fig. 6c), which 

 in the living animal is occupied by a thread of organic 

 matter. 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 silica in a crystalline or crypto-crystalline 

 condition, and is consequently easily distinguished between 

 crossed Nicols when the spicule itself still remains colloidal 

 and consequently isotropic ; generally however the spicule 

 has become crystalline or crypto-crystalline, and when 

 this is the case the axial canals can rarely be detected 

 since they are filled with material in the same condition. 

 Sometimes however the silica of the spicules has been 

 entirely removed, in some cases a hollow cast only remain- 



