OS 



. Diagram of sponge, 

 one of the numerous pores or mou 

 c, a ciliated chamber or pocket 

 osculum 



p, Fig. 29a. A longitudinal section through a 

 ths ; simple calcareous sponge, showing the simple 

 ; os, central cavity; b, showing a single osculum at 

 the top, and the many mouths over the surface. 



Fig. 306. Development of a sponge (Sycon raphanus). A. ripe egg; B. stage with 

 four segmentation-cells; (7, morula stage, with sixteen cells; D, blastospheiv (blastula), 

 with large dark granular cells (gc) at the open pole; E, free-swimming blastula, one 

 half of the body (endodermal) being formed of long ciliated cells, the other (ectoder- 

 mal) of large granular cells. All highly magnified. After Schultze. 



[To face page 44.] 



