150 THE WORLDS LUMBER ROOM. 



The fibres of the skeleton are, for the most part, of 

 uniform size, but here and there one is thicker than the 

 rest, and contains a thorn of flint, called a " spicule." 

 These are rarely found in the sponges sold for toilet pur- 

 poses, but the skeletons of other kinds are frequently 

 strengthened by them. Though usually of flint they are 

 sometimes of carbonate of lime, and though all of micro- 

 scopic size, the variety in their shapes is simply endless. 

 There are fairy fish-hooks, crochet-hooks, stars, and toasting- 

 forks, looking as if they were made of glass of different 

 colours. (Fig. 30.) 



In some sponges the skeleton is entirely composed of 

 spicules of carbonate of lime ; and in many both fibres and 

 spicules are of silica. One of these latter, called " Venus's 

 Flower-basket" (Fig. 31, A), from its great beauty, is a 

 tapering tube curved like a horn, which looks exactly as 

 if woven out of white spun glass, so regular is the network. 

 When alive it is covered with a film of greyish-brown jelly, 

 and lives half-buried in mud, into which it is prevented 

 from sinking by a fringe-like root of glassy spicules. 



Another kind, called the glass-rope sponge, or sea-whip 

 (Fig. 31, B), has long roots of transparent, colourless 

 silica, like threads of glass, some as fine as hair, others 

 as coarse as twine, which are twisted into a coil about a 

 quarter of an inch thick, and some twenty or more inches 

 long. The roots are buried in mud, and the soft sponge 

 grows above, but the whole was thought to look so "un- 

 natural " that, as the first specimens were brought from 

 Japan, they were believed to be artificial productions. 

 Fine sea-whips have, however, been found off the coast 



