SPONGES. 399 



tubular fibres ; but in all existing species, with one or two 

 exceptions, they are solid. These tubular fibres are very 

 commonly filled with portions of iron, which accounts for 

 the colour of many of the remains in flint. 



The Moss-agates, found among the pebbles at Brighton 

 and elsewhere, are flints containing the fossilised remains of 

 sponges. The coloured fibres seen in the Green-jaspers of 

 the East are of the same character. There is reason to 

 believe that most flints were originally sponges ; those 

 from chalk even retain their original form. Recent 

 sponges from the Sussex coast present forms precisely 

 similar to some chalk flints, but it is from sections made 

 sufficiently thin to be transparent, for examination under 

 the microscope, that we learn their true nature and 

 origin. 



Every horny sponge, whilst living, is invested with a 

 coating of jelly-like substance, which can only be preserved 

 by placing the sponge in spirit and water immediately 

 after its removal from its place of growth. Spicula are 

 not exclusively confined to the body of sponges, but occa- 

 sionally form the skeleton of the gemmules, and are situated 

 either on'the external or internal surface of these bodies. 

 A good example of the former kind occurs in the common 

 fresh-water sponge (Spongilla fluviatilis), represented in 

 fig. 216, No. 1, and No. 3. The spicula are very minute 

 in size, and are disposed in lines radiating from the 

 centre to the circumference, the markings on the outer 

 surface of the gemmules being the ends of spicula. In all 

 the young gemmules the spicula project from i>he outer 

 margin as so many spines ; but in process of growth the 

 spines become more and more blunt, until at last they 

 appear as so many angular tubercles. Turkey sponge 

 (Spongia officinalis) is brought from the Mediterranean, 

 has a horny network skeleton rather fine in the fibres, 

 solid, small in size, and light in colour. In some larger 

 specimens there is a single large fibre, or a bundle of 

 smaller ones. In Halichondria simulans the skeleton is a 

 framework of siliceous needle-shaped spicula, arranged in 

 bundles kept together by a thick coat of horny matter. 

 Other species of Halichondria have siliceous spicula 

 pointed at both extremities aca-rate (fig. 212, No 2) ; 



