122 



THE SEA SHORE 



unsupported by harder structures. They are sometimes known 

 collectively as the Silicia, for the greater number of them have 

 skeletons consisting exclusively of siliceous matter, while the 

 so-called horny sponges usually have spicules of silica inter- 

 mingled with the horny substance, and even those which are 



described as having no skele- 

 ton at all sometimes contain 

 scattered spicules of silex. 



As the spicules of sponges 

 are in themselves beautiful 

 objects, and are important to 

 the naturalist, inasmuch as 

 they form a basis for the 

 classification of sponges, it is 

 well to know by what means 

 they may be separated from 

 the animal for microscopic 

 examination. The separa- 

 tion is based on the fact that 

 nitric acid (aqua-fortis) will 

 destroy organic matter while 

 it has not the slightest action 

 on silica. In some of our 

 common horny sponges the 

 fibres are so transparent that, 

 when teased out and placed 

 under the microscope, the 

 siliceous spicules may be seen 

 embedded within them, but 

 the spicules, both in these 

 and the fleshy sponges, may 

 be separated completely from 

 the animal matter by putting 

 a fragment of the sponge in 



a test-tube, covering it with nitric acid, and boiling it for a short 

 time. The tube should then be filled up with water and allowed 

 to stand undisturbed for a time, after which the liquid is poured 

 off gently from the sediment. If the sediment is then put under 

 the microscope on a slip of glass, it will be seen to consist of grains 

 of sand, of which there is always a considerable amount in the pores 

 and cavities of a sponge, and the siliceous spicules. 



FIG. 72. Chalina oculata 



