62 FOSSIL SPONGES. PABTIII. 



the fresh- water sponges may be, a multitude of minute 

 hard yellow bodies are produced in their deeper parts. 

 They consist of a tough coat containing radiating spi- 

 cules like a pair of spoked wheels united by an axle with 

 a pore in its surface. Within this last there is a mass of 

 motionless granular cells, and when put into water the 

 cells come out at the pore and give rise to new sponges. 



Insulated groups of germs covered with cells called 

 swarm-cells seem to form parts of the sponges ; they lie 

 completely within the mass of the living sponge. They 

 have the form of a hen's egg, are visible to the naked 

 eye, and when they come into the water they swim in 

 all directions for a day or two ; become fixed ; a white 

 spot within is enlarged ; and the constituents of young 

 sponges appear. 7 



The generic forms of fossil sponges augment in 

 number and variety from the Silurian to the Cretaceous 

 beds, where the increase is rapid ; but all the sponges 

 which had a stony reticulated form without spicules 

 : passed away with the Secondary epoch, so that the 

 family has no representatives in the Tertiary deposits or 

 existing seas. The calcareous sponges which abound 

 in the Oolite and Cretaceous strata, and attain their 

 maximum in the Chalk, are now almost extinct, or are 

 represented by other families with calcareous spicules. 

 Siliceous fossil sponges are particularly plentiful. In 

 England extensive beds of them occur in the Upper 

 Greensand, and in some of the Oolitic and Carboni- 

 ferous 4 Limestones; and some beds of the Kentish Rag 

 are so full of their siliceous spicules, that they irritate 

 the hands of the men who quarry them. Since every 

 geological formation except the Muschelkalk is found 

 in England, the number and variety of fossil sponges 

 are very considerable. The horny sponges are more 

 abundant now than they were in the former seas. Ac- 



7 Professor Huxley's Lectures. 



