34 



SPONGES 



current canals also become complicated and ramified. The whole 

 canal system may now conveniently be divided into three parts : 

 (1) The incurrent system, from the ostia to the prosopyles of the 

 ciliated chambers ; (2) the chambers themselves ; and (3), the ex- 

 current system from the chambers to the osculum. 



The canal systems of the third type may become highly 

 differentiated and complicated in their several parts. Both in- 

 current and excurrent canals may branch repeatedly and undergo 

 various modifications in different regions. Quite apart from the 

 complications of these systems, three stages of evolution are to be 



in.c. 



exc: 



*>-y%*m 



-C- i:/:sr < ' ^ V^>i i' 



'^j3^iifca^i^***l ' , 

 -*;*<;' sjr^^iti >'. 



** ^ *2Jg| 



m ^cSftill 



; ^s^ S^*| 



'N v tf E ^t f tft 



';') .5a - '^.-^ ^-.T.^.-^f- t " 



A. 



B 



FIG. 45. 



Diagrams of the third type of canal system (eurypylous). A, more primitive, with elongate 

 chambers. B, with rounded chambers, ex.c, excurrent canals. Other letters as in last. 



observed in the relations of the chambers to the incurrent and ex- 

 current systems, by means of which canal systems of the third type 

 can be divided into three sub-types. 



In the first and most primitive sub-type the chambers open 

 directly into the excurrent canals by their wide apertures or 

 apopyles, and receive their water supply direct from the incurrent 

 canals through the prosopyles (Fig. 45, A and B}. A canal system 

 of this type is said to be eurypylous. 



In the second sub-type the opening of the chamber into the ex- 

 current canal is drawn out into a tube, usually not of great length, 

 termed an aphodus (Fig. 46, A, aph). The relations of the chamber 



