SECT. Ill 



PHYLUM AND CLASS PORIFERA 



107 



i/ 



FIG. 79. 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 



F. 8 0.-sycon se latino.m. A portion s .ight,y 



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, 

 dotted; ip. marks the position of three of the groups 

 of inhalant pores at the outer ends of the incurrent 



canals o osculum 



!T 3 l 



through the minute in- 



-nnrPQ nr ncrh'a 

 pOre& Or OStia 



rnpntionpfl a4 



Occurring in groups be- 



, -P , & , . * 



tween tne elevations on 



