CERATOSA KEY TO BRITISH GENERA 



221 



are characterised by a granular ground substance and aphodal 

 chamber system ; the Spongelidae by a clear ground substance and 

 sac-like eurypylous chambers. 



The bath sponge, Uuspongia officinalis, belongs to the Spongidae. 

 The finest varieties come from the Adriatic, the coarser ones from 

 the Dalmatian and North African coasts of the Mediterranean, 

 from the Grecian Archipelago, from the West Indies, aiid from 

 Australian seas. The softer species of the genus Hippospongia 

 also form a source of somewhat inferior bath sponges. 



Among Dendroceratina, Darwinella is unique and tempts to 

 speculation, in that it possesses isolated spongin elements, resemb- 

 ling in their forms triaxon spicules. 



Key to British Genera of Sponges. 



Skeleton calcareous 



Skeleton siliceous 



Skeleton horny, or without free spicules 



Skeleton absent 



chambers 



. 53 



. 55 



3 



4 



Glathrina 

 Leiicosolenia 



5 



Leucandra 



Sycon 

 Ute 



10 

 15 



j'Gastral layer continuous 



■ \ Gastral layer discontinuous, confined to 



/Equiangular triradiate systems present 

 \Triradiate systems all alate 



f Chambers tubular, radially arranged 

 \ Chambers spherical, irregularly scattered 



f Tufts of oxeate spicules at the ends of the chambers 



■ I^Oxeate spicules lying longitudinally in the cortex 



(AU the spicules hexradiate or spicules easily derived from hexradiate 

 type . . . . . • -7 



Some of the spicules oalthrops or triaenes 

 Megascleres uniaxial 



„ TAmphidiscs present 



■ \Amphidiscs absent 



(Booting spicules a well-defined wisp ; four apertures lead into the 

 gastric cavity . . • • Hyalonema thomsoni 



Rooting tuft diffuse ; sponge oval ; osculum single 

 Pheronema carpenteri 



{Sponge tubular, dermal and gastral pinuli absent Euphctella suberea 

 Sponge a widely open cup ; dermal and gastral pinuli present 

 Asconema setuhalense 



(Tetractine spicule, a calthrop or triaene with short rhabdome; 

 microsclere a spined microxea . . Bercitus hucklandi 



Triaenes with fully developed rhabdome . . .11 



