SPONGES 33 



outer surface of the body, which have the appearance of pores, and 

 are often so termed ; but it is obvious from the development that 

 the pores on the surface of the body in this type are not comparable 

 to those of the Oli/nthus, which are represented now by the chamber 

 pores. To avoid confusion it is best to employ a terminology wiiich 

 distinguishes clearly between them, and hence the openings of the 

 incurrent canals may be termed the ostia, Avhile the chamber pores 

 receive the special name of })roso])ijles. Similarly, the wide opening 

 by which the current passes out of the chamber may be termed the 

 apopijle. 



The follo^ving parts can, therefore, now be distinguished in the 

 fully developed canal system of the second type (Fig. 44, B). The 



A 



Ij' osc. \\ 



f^P flc. Mc. 



4 ^ 



tS-.fr 





flc 



af/p. 



\ost. 



B 



Fic!. 44. 



Diagrams of the second tyi)o of canal systpm. A, simple type, with separate radial tubes. 

 B, more complex type, with radial tubes fused and thickened distally to form cortex and in- 

 current canals ; a portion only of the wall is represented, o^l, ostia ; iii.c, incurrent canals ; 

 pr.p, prosopyle ; /?.<■, tiagellated chamber ; nji.j), apopyle. Other letters as in last. 



water enters through the ostia (dermal pores) into the incurrent 

 canals ; thence it passes through the prosopyles (chamber pores) 

 into the ciliated chambers ; and from them it passes by the wide 

 apopyles into the gastral cavity and out through the osculum. The 

 gastral layer, being restricted to the chambers, is discontinuous, as 

 it is in all types of canal system above the first type. 



Third Tifpe of Canal System. — The third type can be derived from 

 the second by a further process of folding of the body wall, giving 

 rise to bays or diverticula of the gastral cavity, into each of which 

 several chambers open together (Fig. 45, A). Thus a system of 

 what are termed excurrent or exhalant canals becomes inter- 

 polated between the chambers and the gastral cavity proper. 

 In correspondence with this addition to the canal system the in- 



