168 THE MICROSCOPE 



scopic study. Dr. Bowerbank said of recent English sponges 

 that after fifty years' experience of them, he frequently found that 

 a guess at the species by external examination, of even the com- 

 monest kinds, was frequently wrong ; but that a section at right 

 angles to the surface under the microscope settled the question 

 with ease and certainty. Different genera of sponges may assume 

 the same form, and diverse forms may belong to the same genus 

 or even species. How much more, then, must microscopic sec- 

 tions be required, in dealing with the damaged and altered 

 remains of fossil sponges ! 



Mr. Sollas describes some of the changes which fossilisation 

 causes at times in Hexactinellid sponges : how crystalline, trans- 

 parent calcite fills up the meshes of the network, and occupies the 

 hexradiate canals of the siliceous fibre, and encloses the fibre, in a 

 few cases, almost as homogeneous and purely siliceous as when it 

 existed in the living state ; but more generally, specimens shew a 

 further change. The siliceous fibre becomes granular, absorption 

 takes place mainly from within outwards in each fibre, and calcite 

 is concurrently deposited. But even in this extreme mineralogi- 

 cal change, the original structure is not obliterated. The calcite 

 which fills the internal canal and the interspaces of the meshes is 

 transparent and usually colourless, or with faint yellowish tinge ; 

 while that which replaces the siliceous fibre is, by reflected light, 

 of a milky-blue colour, and by transmitted light, brownish, less 

 transparent, and granular, with dark spots. And thus, while the 

 fundamental spicule has become absorbed and its hollow cast 

 filled with crystalline calcite, and the same material has replaced 

 the siliceous fibre, and the sarcode between the meshes ; while, in 

 fact, the whole of the metamorphosed net consists of one mate- 

 rial, carbonate of lime, the structure is left as definitely recorded 

 as in a sponge, with its natural composition only just dead. 



Other and further changes at times take place, and when the 

 sponge is partly fossilised by calcite externally and sifica internally, 

 the central canal is often once more absorbed, and again, as in its 

 primary state, filled with silica. This may be changed, silica again 

 taking possession of the form of the fibre, and minute granules of 

 iron pyrites taking up the form of the central canal. The remains 

 of the first-known sponge, the Cambrian Protospongia^ was pro- 



