PORIFERA. 29 



ing ; in others its place is taken by another mineral, as for 

 instance by calcite in the sponges from the Lower Chalk 

 of Dover and Folkestone, by iron pyrites in Protospongia 

 from the Menevian Beds of St Davids, by iron peroxide in 

 the sponges of the Upper Chalk of the South of England, 

 by glauconite in those from the Upper Greensand. 

 Obviously then silica in the colloidal state in which it 

 occurs in recent sponge spicules is anything but a stable 

 substance, thus differing widely from its crystalline and 

 crypto-crystalline condition. 



The spicules of the calcareous sponges are usually 

 smaller than those of the siliceous forms, and their material 

 is not in an isotropic state, each one being optically a crys- 

 tal of calcite but having the external form and internal 

 structure of a spicule, consequently in polarised light they 

 are readily distinguished from the siliceous forms. 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, it is only 

 in rare cases that this is replaced by silica, but at the 

 same time the external form of the individual calcareous 

 spicules has very often disappeared. 



The canal-system is indicated in the skeleton of both 

 recent and fossil forms, by spaces in the spicular frame- 

 work, but these spaces represent only the larger canals, 

 the smaller existing in the soft parts alone. 



Reproduction in the sponges takes place by budding 

 or gemmation, and sexually. 



The Porifera may be divided into the following nine 

 orders. 



ORDER 1. MYXOSPONGLE. Sponges with no skeleton 

 or occasionally a few isolated siliceous spicules. Not 

 known in the fossil state. 



