18 



zooLoay 



After invaginution the blastula becomes a gastrida and 

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

 Sponges may liave horny spicules as in those in domestic nse, 

 or calcareous or silicions sjiicules. Fig. 17 represents a 

 fine silicious sponge from the West Indies. The most 



Fig. 17. — Fheronenia 4nnrp liilf mturil size, with stellate and anchor-like 

 siiicules, much enlarged. 



beautiful of all silicious sponges is the A^enus' flower-basket 

 {Euplectellum asjiergiUum), which lives anchored in tlie 

 mud at the depth of about 10 fatlioms, near the Pliiliiijiino 

 L-lands. 



A sponge called the Cliona bores into shells, causing them 



* Bee llacelvel's Die Kiilliseliwilmme. t! vols. 1873. 



