190 THE MICROSCOPE. 



The Moss-agates, found among the pebbles at Brighton and else- 

 where, are flints containing the fossilised remains of sponges. The 

 coloured fibres seen in the GVeew-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. Eecent 

 sponges from the Sussex coast present forms precisely similar to some 

 chalk flints, but it is from sections made sufficiently thin to be trans- 

 parent, for examination under the microscope, that we learn their true 

 nature and origin. 



Every horny sponge, when living, was 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 

 occasionally form the skeleton of the gemmules, and may be 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 Plate IV. Nos. 9 and 13. 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 the 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, or Spongia officinalis, is brought from the Medi- 

 terranean, has a horny network skeleton rather fine in the fibres, 

 solid, small in size, and light in colour (Plate IV. No. 1.) In some 

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

 ones. 



In Halichond/ria 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 — acerate (Plate IV. No. 7) ; while the spicula of some are 

 round at one end and pointed at the other — acuate (see No. 7) ; and 

 also spicula round at one end, the former being dilated into a knob — 

 spinulate ; some have a spinous surface. 



From the South Seas specimens are found having spicula with both 

 ends rounded ; cylindrical ; some curved, others straight. 



In an unclassed genus, the spicula are discovered rather cylindrical 

 in form, pointed at both ends, having the surface covered with spines 



