CHAP. Ill 



POEIFERA 



41 



stitute it become invaginated into the posterior half, and the blasto- 

 coele is thus reduced to a mere slit. The cells forming the edge 



B 



gr 



FIG. 18. Two stages in the fixation of the larva of s >'//<''"'"' m ritjilnnnts. (After Sehulze.) 

 A, the flagellated cells are just retreating into the interior. B, the larva lias assumed the form of a 

 hemispherical cup and is attached by amoeboid processes of the outer granular layer. Seen in optical 

 section, fl, flagellated cells ; gr, granular cells ; ;i, porocytcs (?). 



of the cavity of invagination are granular, and when the ciliated 

 cells become invaginated, these granular cells extend inwards 



FK;. 19. An early stage in the metamorphosis of the Ascon stage of Sycandra 



into the adult. (After Maas.) 



A, external view of young sponge. B, diagrammatic longitudinal section of the same to show the 

 gradual displacement of collar cells by granular cells. C, a small portion of such a section further 

 enlarged, r/i, radial flagellated chainber ; fl, flagellated cells; gr, granular cells (in C the reference 

 line points to a spot where the granular cells are migrating inwards) ; os, osculum ; p, inhalent pore ; 

 PU, paragaster. 



along the substratum, and floor the cavity of the invagination 

 (Fig. 18); they also extend outwards in irregular tongue-like pro- 



