METAZOA 45 



(i.) Hyalospongiae. — Skeleton of siliceous spicules, which are 

 usually sexradiate, isolated, or fused into a trellis-work. 

 Usually deep-sea forms. Many fossils. Ex. Hyalonema, 

 whose spicules may reach a length of two feet ; Euplec- 

 tella, or Venus's flower-basket. 



(ii.) Spiculispongiae. — Skeleton absent in a few forms ; in- 

 the great majority consisting of siliceous spicules, 

 usually independent, hut sometimes articulated together, 

 or united hy organic material. One of the sub-orders, 

 the Myxospongiae, is devoid of skeleton. Ex. Hali- 

 sarca, Oscarella. In the Tetractinellidae, one of the 

 largest subdivisions, the spicules are to a great extent 

 four-rayed. Ex. Geodia, Tetilla. 



(iii.) Cornacuspongiae. — Skeleton of uniaxile spicules united 

 hy spongin, or of spongin. Inhabit the sea, brackish or 

 fresh water. Ex. Euspongia, the sponge of commerce ; 

 Velinea, Spongilla (the freshwater sponge), etc. 



