CANAL-STRUCTURES. 49 



of canals ; one, the incurrent system, leading from the surface to the interior of the 

 Sponge, and the other or excurrent passing from the interior and opening out at 

 the surface or into the cloaca. The incurrent canals (Fig. 1, in.) are, as a rule, 

 much smaller than the excurrent ; they commence at the surface by small circular 

 apertures which are distributed irregularly, and extend into the sponge- wall either 



wC/. 





FlG. 1. — Siphonia tulipa, Zittel. A vertical median section through the Sponge, showing the cloaca 

 (cl.), the fine incurrent canals (in.), and the excurrent canals (ex.) opening into the cloaca. From the 

 Upper Greensand at Warminster, Wilts. Natural size. 



at right angles or obliquely to the surface. The apertures of these incurrent 

 canals have usually been named "pores," but, as 1 Dr. Holl has already remarked, 

 they are not analogous to the pores in living Sponges, which are minute apertures 

 in the soft surface membrane of the Sponge, and could not, therefore, be represented 

 in the skeleton. The apertures in the fossil Sponge rather correspond to the open- 

 ings of the incurrent canals in the recent forms, and may be either termed incur- 

 rent-canal apertures, or, as Sollas names them, ostia. The excurrent canals 

 (ex., Fig. 1) are much more prominent than the incurrent, and less numerous. 

 They open out by circular apertures, either at irregular intervals on the upper 

 surface of the Sponge, or they converge to the infundibuliform or tubular 

 cavity in the centre of the Sponge, which is known as the cloaca or cloacal tube 

 (cl., Fig. 1). Sometimes the excurrent canals terminate at the surface in papil- 

 liform projections, with the aperture at the summit. Their apertures are 

 known as oscules or vents, but the former term has sometimes also been employed 

 for the cloaca itself. 



1 " Notes on Fossil Sponges," ' G-eol. Mag.,' 1872, vol. ix, p. ;346. 



G 



