SECT. Ill 



PHYLUM AND CLASS PORIFERA 



97 



innumerable elevations of a polygonal 

 whole surface and are separated off 

 from one another by a system of de- 

 pressed lines. In these depressions 

 between the elevations are to be de- 

 tected, under the microscope, groups 

 of minute pores the inhalant po r r<*. 

 At the free end of each of the cylin- 

 drical branches is a small but distinct 

 opening, surrounded by what appears 

 like a delicate fringe. When thq. 

 branches are bisected longitudinally 

 (Fig. 70), it is found that the terminal 

 openings (0) lead into narrow passages, 

 wide enough to admit a stout pin, 

 running through the axes of the 

 cylinders ; and the passages in the 



shape, which cover the 



Fio. 70. Sycon gelatiaosum. A portion slightly 

 magnified; one cylinder (that to the right) bisc, i, J 

 longitudinally to show the central paragastric cavity 

 opening on the exterior by the osculum, and the 

 position of the incurrent and radial canals ; the 

 former indicated by the black bands, the latter, 

 dotted ip. marks the position of three of the groups 

 of inhalant pores at the outer ends of the incurrent 

 canals ; o. osculum. 



VOL. I 



Fir.. GO. Sycon gelatinosum. 



Entire sponge, consisting of a 

 group of branching cylinders 

 (natural size). 



interior of the various 

 branches join where the 

 branches join the pas- 

 sages thus forming a 

 communicating system. 

 On the wall of the 

 passages are numerous 

 line apertures which re- 

 quire a strong lens for 

 their detection. The 

 larger apertures at the 

 ends of the branches 

 are the oscula of the 

 sponge, the passages the 

 paragastric cavities. If 

 a living Sycon is placed 

 in sea-water with which 

 has been mixed sm in- 

 carmine powder, it will 

 be noticed that the 

 minute particles of the 

 carmine seem to be at- 

 tracted towards the sur- 

 face of the sponge, ami 

 will often be seen to 

 pass into its substance 

 through the minute in- 

 halant pores already 

 mentioned as occurring 

 in groups between the 

 elevations on the outer 

 II 



