Sponge- fauna of Norway. 405 



The canals, especially the incurrent ones, are ridged trans- 

 versely by circular rugae, which are simply thin lamellar 

 extensions of the wall, composed of epithelium and a small 

 quantity of a tissue containing numerous very fine fibrilhe, 

 which are arranged concentrically in each ruga and are 

 si ightly more abundant along its edge than elsewhere. The rugae 

 are often so greatly developed as to form iris-like diaphragms 

 extending almost halfway across the canal ; and as the canal 

 is also constricted around the origin of the diaphragm, it thus 

 becomes divided into a series of bladder-like compartments. 

 This vesicular character occurs in many other sponges, but in 

 none so markedly as in Thenea Wallichii, which will be 

 described subsequently. In Isopis the vesicular character is 

 most pronounced in the incurrent tubes, if not confined to 

 them, and the rugas or diaphragms likewise are chiefly charac- 

 teristic of these tubes, occurring in all, from the largest down 

 to those having a diameter of only -^Iru inch or less ; in the 

 excurrent tubes they are never so numerous nor extended so 

 far across the canal, nor do they occur in tubes of such small 

 diameter as in the incurrent system ; it appears to me that 

 they never occur in excurrent tubes unless of considerably 

 over yJj-jy inch diameter. 



The physiological explanation of this difference in structure 

 between the excurrent and incurrent tubes appears to lie in 

 the fact that the water expelled into the former is under a 

 slight excess of pressure, which is sufficient to keep them 

 Avidely open ; it is propelled by a vis a tergo. The water in 

 the incurrent tubes, on the contrary, is drawn through them 

 by a vis a fronte, and is thus under a slightly diminished 

 pressure ; they would therefore tend to be compressed by the 

 water in the surrounding tissues ; and it is possibly to prevent 

 this that their walls are strengthened by the concentric rugse. 



Ciliated Chambers (PI. XVII. fig. 23). — These organs are 

 almost spherical in form and 0*00l inch in diameter; they 

 consist of a structureless membrane, covered on the inner 

 surface by roundish nuclei, surrounded by granular proto- 

 plasm, and disposed at very regular distances apart. Cilia 

 proceed from these nucleated patches, radiating from the walls 

 towards the centre of the chamber. They thus clearly repre- 

 sent, as far as they could be preserved, the collared cells of 

 other sponges. 



As previously mentioned, the ciliated chambers everywhere 

 surround in close proximity the walls of the whole of the incur- 

 rent canals, large and small alike (IM. XVII. rig. 27). Short 

 narrow canals, usually about 0'0006 to 0*0009 inch long and 

 0*00025 inch in diameter, open abruptly into them and connect 



