SPONGES 



35 



to the incurrent canal remain as before. A canal system of this 

 kind is termed aphodal. 



In the third stage the chamber has, as in the last, an aphodus, 

 and in addition a delicate canalicule termed a prosodus interpolated 

 between the chamber and the incurrent canal (Fig. 46, B}, and 

 derived, probably, by elongation of a prosopyle. Canal systems of 

 this kind are termed diplodal. 



Thus in the most highly differentiated type of canal system, 

 the following series of parts can be distinguished : ostia, incurrent 

 canals, prosodi, ciliated chambers, aphodi, excurrent canals, gastral 



fU 



w, 



exc. 



Pio. 40. 



Third type of canal system. A, aphodal ; B, diplodal. aph, aphodus ; prs, prosodus. Other 

 letters as in last. 



cavity, and osculum, and to these may be added further complica- 

 tions of the incurrent system which will be described when dealing 

 with the canal system in the different groups. 



The diplodal canal system is regarded by some authors as con- 

 stituting a fourth type of equal value with the other three. 



Osculum, Gastral Cavity, and Pores. The gastral cavity, properly 

 speaking, extends up the exhalant canals and includes the cavities 

 of the chambers. This is obvious from the development of the 

 canal system that has just been traced. It is more usual, however, as 

 well as more convenient in most cases, to distinguish the cloacal cavity 

 which opens by the osculum, and into which the exhalant canals unite to 



