CCELENTERATA 



59 



fibers, but it consists of spongin, and there are no spicules ot 

 silica (Fig. 39). The skeleton is most familiar as the hoi se- 

 sponge, the common bath-sponge, and the silk-sponge. These 

 sponges do not grow at great depths, — they are most common 



FlG. 42 bis. Sycon raphanus, development, o, ovum ; b, c, ovum segmented — b, as seen 

 from above, <-, lateral view; d, blastula; e, amphiblastula ; f, beginning of invagina- 

 tion ; g, gastrula attached by its oral surface ; //, /, young sponge — h, lateral view ; /, a^ 

 seen from above. (After Schulze, from Parker and Haswell's Text-book.) 



in water less than 150 meters deep, and occur in rounded 

 colonies. The commercial sponges generally live in water 

 less than 60 meters deep, on rocky or coral shores, and are 

 secured either by clivers or by means of long hooks or tri- 



