II 



POEIFERA 



Order 2. Hexactinellidse. 



61 



Siliceous needles, isolated or bound together by masses of silica into a continuous 

 firm framework, til-axial. Flagellate chambers cylindrical, placed radially, similar 

 to the radial tubes of Sycandra. Most forms fossil. Living: Euplcctclla, Hyaloncma. 



FIG. 50. Skeleton of a horn sponge attached to a stone, o, Oscula. 



Order 3. Spiculispongise. 



Skeleton consisting of independent siliceous spicules of various kinds, rarely 

 wanting. The spicules are often bound together into bundles by an organic sub- 

 stance, or form firmly connected skeletons interlocking by means of knotty out- 

 growths ; they are, however, never cemented together by siliceous masses. Geodia, 

 Plakina, Chondrosia, Oscarella, and Halisarca (without skeleton), Tethya, Tuberella, 

 Subcrites. 



Order 4. Halichondrina. 



Skeleton composed of siliceous spicules, chiefly uniaxial, cemented together by a 

 more or less horny substance (spongin). Halichondria, Reniera, Spongilla (in fresh 

 water) Myxilla, Clathria. 



