SILICEOUS SPONGES 143 



found along with flints are of a very different nature to 

 the familiar bath-sponge. That, in the form we are 

 acquainted with it, is the skeleton of a particular species 

 which lives in the Mediterranean and the Caribbean Sea. 

 It is formed of a network of delicate elastic horny fibres, 

 and to their elasticity it owes its wonderful softness. 

 But the sponges we are concerned with possess, like 

 the great majority of their class, a skeleton formed of 

 hard mineral matter, often of so harsh a texture that 

 no one would think of applying it to the skin, unless as 

 a counter-irritant. The beautiful Venus's flower-basket 

 (Euplectella) is an example of one of these sponges (Fig. 

 39), and the nearest familiar representative of one of 

 the forms (Ventriculites) sometimes found associated 

 with flints. The living Euplectella is fished up by the 

 Japanese from the deep sea ; it is considered by them 

 a very appropriate wedding present, partly because it 

 rather commonly contains a pair of crab-like inhabitants, 

 which, being caged in so that there is no chance of escape, 

 are supposed to symbolise a state of conjugal felicity. 



The mineral matter forming the skeleton of this group 

 of sponges is siliceous ; it consists of silica, the same 

 material which forms the substance of flint, but in this 

 case in chemical combination with water, so that it is 

 known to the chemist as as silica hydrate or silicic acid, 

 and may be represented by the symbols Si 5 Oi H 2 ; it 

 is familiar to all of us in another form as opal. Opal 

 plays a large part in the organic world, the skeletons of 

 those lowly plants the diatoms are formed of it, it occurs 

 in grasses, and forms the tabasheer which is sometimes 

 found accumulated in the joints of bamboos; in the 

 animal kingdom it not only forms the hard parts of the 

 vast majority of sponges, but the beautiful and diversified 

 tests of the radiolaria, and the rasping teeth of some 

 kinds of sea-snails. 



