ii8 TOILERS IN THE SEA. 



left out of the question, as the spicules are confined 

 to a few species. 



The skeleton of the horny or keratose sponges, 

 such as the toilet sponge, is built up of horny fibres 

 which branch and coalesce, and are interwoven into 

 a kind of basket-work, sometimes strengthened by 

 grains, sand, and other extraneous matter, or some- 

 times by a few spicules, but the most essential ele- 

 ment is the horny fibre. In the calcareous sponges 

 carbonate of lime, in the form of spicules combined 

 with membrane, is the skeleton element ; but in the 

 siliceous or flinty sponges different forms and com- 

 binations of spicules cemented together form the 

 skeleton (fig. 17). 



" Sometimes the skeletons assume the shape of a 

 beautiful regular or irregular reticulation, composed 

 either of a nearly single series of elongate forms of 

 spicules, cemented firmly together at their apices by 

 keratode (which is the principal substance in the 

 skeleton of the horny sponges), or by numerous spi- 

 cules similarly cemented together, forming a strong 

 and complicated fasciculated thread of reticulations. 

 In other cases there is no reticulated structure, but 

 the spicules are arranged in elongated compound 

 bundles, which radiate from either the base or cen- 

 tral axis of the sponge, whilst in others the reticu- 

 late and the radial system both enter into the struc- 

 ture of the skeleton, a modification of the network 



