18 
ZOOLOGY 
After invagination the blastula becomes a gastrula and 
fixed to the bottom, the spicules growing as in Fig. 15, 
Sponges may have horny spicules as in those in domestic use, 
or calcareous or silicious spicules. Fig. 17 represents a 
fine silicious sponge from the West indies. The most 
Fig. Yi.—Fheronema AnncB, half natural size, with stellate and anchor-like 
spicules, much enlarged. 
beautiful of all silicious sponges is the Venus' flower-basket 
{Euplectellum aspergillum), 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 Haeckers Die Kalkschwamme. 3 vols. 1872. 
