PORIFERA. 35 



bonate of lime (fig. 6, j); usually much smaller and less 

 varied in form than those of the siliceous sponges ; there 

 are three kinds, the simple uniaxial, the three-rayed, and 

 the four-rayed ; they are never fused with one another, 

 and they are either arranged close together so as to form 

 fibres, or they are loosely distributed. The earliest forms in 

 Britain occur in the Carboniferous rocks of Fifeshire. 



Peronella. Cylindrical, simple or branched ; central 

 cavity tubular and extending from the summit to the 

 base. Walls thick and with no definite canals, the current 

 of water having probably passed through the irregular 

 spaces between the spicular fibres. Spicules three or four- 

 rayed forming anastomosing fibres. Carboniferous (pos- 

 sibly also Devonian) to Cretaceous ; most abundant in the 

 Jurassic and Cretaceous. 



Distribution of the Porifera. 



The Sponges are all aquatic, and, with the exception of 

 the Monactinellid genus Spongilla, and its allies, all marine. 

 Fhey are found in the seas of all parts of the world, many 

 jf the genera and species having a very wide distribution. 

 AJ1 the orders except the OctactinellidaB and the Heterac- 

 iinellidse are still living. The Lithistids are found mainly 

 between 10 and 150 fathoms, but some species occur 

 it greater depths. The Hexactinellids are deeper water 

 forms than the Lithistids, being found down to a depth 

 )f 2900 fathoms, but they are most abundant between 

 100 and 200 fathoms. The CalcispongiaB are mainly 

 shallow water forms. 



The fossil forms are comparatively rare in the Palaeozoic 

 ocks, until we reach the Carboniferous ; and throughout 



32 



