SECT. Ill 



PHYLUM AND CLASS PORIFERA 



107 



FIG. 70. Sycon gelatinosum. 



Entire sponge, consisting of 

 a group of 'branching cylin- 

 ders (natural size). 



innumerable elevations of a polygonal shape, which cover the whole 



surface and are separated off from 

 one another by a system of depressed 

 lines. In these depressions between 

 the elevations are to be detected, under 

 the microscope, groups of minute 

 pores the ostia or inhalant pores. 

 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 the 

 branches are bisected longitudinally 

 (Fig. 80), it is found that the terminal 

 openings (o) lead into narrow passages, 

 wide enough to admit a stout pin, 

 running through the axes of the 



cylinders ; and the passages in the 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 

 fine 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 some 

 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, and 

 will often be seen to 

 pass into its substance 

 through the minute in- 

 halant pores or ostia 

 already mentioned as 

 occurring in groups be- 

 tween the elevations on 



FIG 80 Sycon gelatinosum. A portion slightly 

 magnified ; one cylinder (that to the right) bisected 

 longitudinally to show the central paragastric cavity 

 opening on the exterior by the osculum, and the 

 position of the incurrent and radial canals ; tne 

 former indicated by the black bands, the latter, 

 dotted ; ij>. marks the position of three of the groups 

 of inhalant pores at the outer ends of the incurrent 

 canals ; o. osculum. 



