54 



HALF AN HOUR WITH SPONGES. 



Fig. 16. 



Osculse of Sponge. 



into three great groups, according to the nature of 

 their framework or skeleton. We have spoken of 

 the spicules, or needle-shaped crystals, which enter 



so largely into the 

 structure of this skele- 

 ton, and are also found 

 .dispersed through the 

 sarcode. Well, these 

 spicules differ very 

 considerably in their 

 mineral nature, some 

 being composed of 

 lime, and others of 

 silex. This difference 

 at once indicates a 

 difference in the animals capable of secreting the two 

 minerals. And yet we are totally in the dark as to 

 how either is secreted. The three divisions referred 

 to are the Keratose, Silicate, and Calcareous. The 

 first includes all the sponges whose framework is of 

 a horny texture, as in the common washing sponge. 

 This group, however, merges in both the others by 

 occasionally having lime and silex spicules dispersed 

 through it. You would find it difficult enough to 

 wash with a genuine silicated sponge ! Indeed, it 

 requires to be handled as gently^as possible, other- 

 wise the fine, needle-shaped spicules will pierce the 

 skin unawares, and produce an irritation for a long 

 time afterwards. The silicated sponges include 

 the most highly organised and ornamental of the 



