PORIFERA. 109 
In these four types there will be noticed a progressive in- 
crease in bulk of the body of the sponge and a progress of the 
choanocytic areas from within outwards. Numerous transi- 
tion types are found, and these types are distributed quite 
indiscriminately throughout the classes or orders. 
Fig. 44.—TRANSVERSE SECTION OF A RHAGON. ' (Diagrammatic. ) 
Sponges fall into two well-defined classes—(1) CALCAREA, 
(2) Non-CALcaREA. 
1. CALCAREA.—The Calcarea all have a_ calcareous 
skeleton and the collared-cells are much larger than those of 
the next class. They are usually Ascons and Sycons, and in 
many points they are simpler and more primitive than the 
Non-calcarea. 
2. Non-Catcarga.—lIn these the skeleton consists of 
ceratin (horny) fibres (with or without spicules), siliceous 
spicules which may fuse, or there may be no skeleton. The 
collared-cells are minute and the canal system is mostly 
complex. 
