Sponge-fauna of Norway. 



403 



These canals are all exceedingly well defined, and all but the 

 very smallest are provided with a distinct wall, which is 

 transversely ridged by concentric rugse. Short narrow cana- 

 liculi lead from the walls of these incurrent canals, main 

 trunks, branches, and twigs alike, and open abruptly into the 

 surrounding ciliated chambers. At a point generally opposite 

 that at which the incurrent canaliculus enters, the ciliated 

 chamber is gradually produced into an excurrent canaliculus, 

 which is somewhat wider and less well defined than the corre- 

 sponding incurrent vessel. The excurrent canaliculi join gra- 

 dually together to form a trabecular tube, which, joining with 

 others of a similar character, at length lead into a large canal 

 with very definite and transversely rugate walls. This canal 

 finally opens into an excurrent chone through a sphincter, and 

 so communicates with the exterior. 



There is a great difference in the way in which the ultimate 

 canals of the excurrent and incurrent tubes are connected with 

 the larger canals : in the former, as previously mentioned, 

 the junction is gradual, the ultimate canals enlarging a little 

 towards the point of junction, and then flowing together at an 

 acute angle ; in the latter the ultimate branches are given off 

 abruptly and, remaining of about the same diameter, end 

 abruptly ; they also make rather a right than an acute angle 

 with the larger, branches. The figures of the annexed woodcut 

 show the difference in character of the ultimate canals of the 

 two systems very plainly. 



1. The ultimate end (u) of an incurrent canal, proceeding from the 



Senultimate branch (p), which is vesicular and provided with 

 iaphragms to the end (X 70). 



2. The ultimate ends of an excurrent tube gathering to form a penulti- 



mate trabecular tube, which has been cut across transversely. 



