18 



ZOOLOGY 



After invagination the blastula becomes a gastrula and 

 fixed to the bottom, the spicules growing as in Fig. 15, E.* 

 Sponges may have horny spicules as in those in domestic use, 

 er calcareous or silicious spicules. Fig. 17 represents a 

 fine silicious sponge from the West Indies. The most 



V* 



Fig. 1?.— 1 'her one ma Annas, half natural siz^, with stellate and anchor-like 



spicules, much enlarged. 



beautiful of all silicious sponges is the Venus' flower-basket 

 {Eaplectellum asperg ilium), which lives anchored in the 

 mud at the depth of about 10 fathoms, near the Philippine 

 Islands. 

 A sponge called the Cliona bores into shells, causing them 



* See Haeckel's Die Kalkschwamme. 3 vols. 1872. 



