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 

 choajiocytic areas from withi7i outivards. 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.) 







- Exhalent Canal. 





£9^ 







* Chamber 

 lined with 

 C h o a n o- 

 cytes. 



Pore. 



Prosopyle. 

 Mesogloea. 



Sponges fall into two well-defined classes — (i) Calcarea, 

 (2) Non-Calcarea. 



I. 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-Calcarea. — In 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. 



