SPOKGIAEIA. 213 



PIEST DIVISION. 

 RADIATA, Cuvier. CYCLO-KETJBA, Grant. 



Body, circular or rayed ; skeleton, simple or complicated ; 

 nervous system, when present, in the form of a knotted 

 filament, encircling the entrance to the stomach. It com- 

 prises five classes : 



1. Spongiaria. 



2. Polypi/era. 



3. Infusoria. 



4. Foramenifera. 



5. Echinodermata. 



The SPONGIAEIA are among the lowest forms of animal 

 life. They are composed of a horny framework, invested 

 with a simple gelatinous tissue, and furnished with vibratile 

 cilia, for causing currents of water to flow through their 

 porous structure; the horny network is consolidated with 

 silicious or calcareous spiculae. The spongiaria remain 

 rooted to rocks at the bottom of the sea, or hang like 

 living stalactites from the vaulted arches of submarine 

 caves ; or their delicate vegetable forms droop in endless 

 variety from the shelving edges of rocks exposed to the 

 washing of the surge. They are reproduced by small 

 gemmae, covered with cilia, which are free organisms during 

 the first period of their existence. We form three orders 

 of this class : the first have silicious spiculae, the second cal- 

 careous spiculae, and the third have a horny network without 

 either. Possil sponges are found in most strata, either 

 entire or decomposed into spiculae. Most of the extinct 

 species are fossilised with silex, which permeated their struc- 

 ture in a fluid state, as it has either enveloped the body of 

 which it retains a cast, or has flowed into all the textures 

 of the organism and preserved a silicious model of its 

 anatomy. To account for the immense accumulation of 

 flints in the upper beds of the white chalk is one of the un- 

 solved problems of geology. These flints nearly all contain 

 the remains of fossil spongiaria. The fibrous internal struc- 



