106 



ZOOLOGY 



innumerable elevations of a polygonal shape, which cover the whole 

 surface and are separated oft' 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. 79), 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 



Sycon gelatinosum. 



— Entire sponge, consisting of a 

 group of branching cylinders 

 (natural size). 



tid. (J. -Sycon gelatlaosunj.— 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; the 

 former indicated by the black bands, the latter 

 dottediip. marks the position of three of the groups 

 of inhalant pores at the outer ends of the incurrent 

 canals ; o. osculum. 



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 



