Sponge-fauna of Norway. 249 



which place it in free communication with the excurrent 

 trunks. These visible openings, however, are but one or two 

 out of a great number unseen, and which are not seen because 

 they are closed by sphincters ; they can readily be made out, 

 however, in sections by transmitted light. 



We shall recur to them in describing the histology of the 

 sponge in detail ; it is sufficient to state now that the excurrent 

 trunks, which break up into small canals in the interior of the 

 sponge, communicate with the oscular tube by means of sphinc- 

 ters, and thus can be shut off from or put into communication 

 with the exterior as circumstances may determine. The in- 

 current canals can best be studied in thin sections (PI. X. 

 fig. 6, and PI. XII. fig. 34) ; the pores of the dermis lead into 

 chones, which open each by a sphincter into the subcortical 

 crypts; from the floor of each crypt a cylindrical tube of sharply 

 defined outline (PI. X. fig. 6, i, and PI. XII. fig. 34) extends 

 downwards for a variable distance into the mark, and, branch- 

 ing below like a bronchus, ends in fine canaliculi. Its walls 

 are more or less finely perforated by openings from which 

 minute canaliculi proceed. Lying parallel with these incur- 

 rent tubes and between them are others of a different character ; 

 they are generally wider, less regular in form, with more 

 widely perforated walls, and are occasionally traversed by 

 an irregular trabecular network (PI. X. fig. 6, e, and PI. XII. 

 fig. 34). From the perforations in their walls canals proceed, 

 which, after branching once or twice, and sometimes anasto- 

 mosing, end in fine canaliculi. The position of these excur- 

 rent tubes with respect to the incurrent tubes is inverse ; i. e. 

 their open extremity is turned towards the centre of the sponge, 

 their more or less closed end towards the rind, while the 

 incurrent tabes lie with the closed end towards the centre and 

 the open end towards and in free communication with the 

 crypts. The floors of the crypts open into narrow short canali- 

 culi like those proceeding from the incurrent canals ; and in 

 both cases these fine canaliculi open, somewhat abruptly, into 

 ciliated chambers, the outflow-canals from which constitute the 

 canaliculi of the excurrent tubes. These excurrent tubes, the 

 primary twigs of the branched excurrent system, communicate 

 with larger canals, which run concentrically with the exterior 

 surface of the sponge (PI. X. fig. 6, c). From these concen- 

 tric canals other canals with trabecular walls proceed and 

 extend deeper into the mark (PI. X. fig. 6, e'), branching till 

 they end in fine canaliculi. These canaliculi end in ciliated 

 chambers, which are connected by shorter canaliculi with other 

 tubes resembling in general character the primary incurrent 

 canals. At first sight the representation of the canal-system 



