PORIFERA 37 



Menevian Beds of St David's, by iron peroxide in the 

 sponges of the Upper Chalk of the south of England, and 

 by glauconite in some from the Upper Greensand. 

 Obviously then, the colloidal silica of recent sponges is 

 anything but a stable substance, thus differing widely 

 from crystalline and crypto-crystalline silica. 



The spicules of the calcareous sponges are usually 

 smaller than those of the siliceous forms, and their material 

 is not in an isotropic state, but each spicule possesses the 

 optical characters of a crystal of calcite ; consequently in 

 polarised light these spicules are readily distinguished 

 from unaltered siliceous forms, which appear dark between 

 crossed Nicol's prisms. Then again the fossil calcareous 

 spicules have undergone much less chemical change than 

 the siliceous ones ; generally they are still composed of 

 carbonate of lime, for it is only in rare cases that this is 

 replaced by silica. The external form of the individual 

 calcareous spicules is, however, often less well preserved 

 than in the case of siliceous spicules. 



The forms of sponge spicules, both megascleres and 

 microscleres, are very varied, but they can be shown to be 

 modifications of a small number of types or fundamental 

 forms. The spicules, on account of the constancy of their 

 characters, are of great importance in the classification of 

 sponges. 



The canal-system is indicated in the skeleton of both 

 recent and fossil forms, by spaces in the framework of 

 spicules or spongin, but these spaces represent only the 

 larger canals, the smaller existing in the soft parts 

 alone. 



Reproduction in the sponges takes place by budding 

 and by the production of ova and spermatozoa. 



Various classifications of the sponges have been pro- 



