Ill 



PHYLUM AND CLASS PORIFERA 



125 



Mast ula () that the larval sponge becomes free. At a later 

 stage the flagellate cells become partly overgrown by the granular 

 cells, the latter eventually giving rise to the ectoderm of the 

 adult, while the former become the flagellate collared cells. The 

 larva becomes fixed by one side, and soon assumes a cylindrical 



Fio. 94. Development of Sycon raphanus. a, ovum ; b, c, ovum segmented 6, as seen from 

 above, c, lateral view ; d, blastula ; e, amphiblastula ; /, commencement of invagination ; 

 g, larva attached by its oral face ; h, i, young sponge h, lateral view ; i, as seen from above. 

 (From Sollas, after Schulze.) 



form (Fig. 94, h, i}. An aperture which is developed at the free 

 end becomes the osculum, and small perforations in the sides of 

 the cylinder form the inhalant apertures. As the wall of the 

 cylinder increases in thickness by the growth of the mesogloea, the 

 radial canals are formed, the endoderm extending into them and 

 its cells becoming flagellate. 



