A GUIDE TO THE SPONGES 



239 



closely akin to silk in chemical composition. It is secreted by 

 the mesoderm and is arranged so as to be a supporting basis to 

 the layers of cellular tissue composing the soft parts of the animal. 

 The fibers are of two kinds, — first, a set of long stout principal 

 ■fibers (see Fig. 11), from ^ to | mm. in diameter, radiating from 

 the base of the sponge to its surface, and secondly, a complicated 

 network of fine connective fibers interlacing between the principal 

 fibers and supported by them. The connective fibers are ex- 

 tremely delicate, having a diameter of only .01 to .02 mm. and with 

 meshes scarcely as large as their diameter. Grains of sand are 

 often found imbedded in the principal fibers, in some cases 

 forming a considerable part of the skeleton, in others the entire 

 substance. The spongin fiber is made up of a soft central core 



FIG. 13— SOME TYPICAL FORMS OF SPONGE SPICULES 



or medullary axis, surrounded with successive layers of the spongin 

 substance. The classification of the homy sponges is based 

 upon the minute characters of the network. A few sponges of 

 small size have no skeleton at all, being supported by whatever 

 rigidity their tissues may possess, but with these exceptions all 

 except the homy sponges have skeletons made up of spicules 

 instead of fibers. These are small needle-like bodies composed 

 of either carbonate of lime or silicon. The latter is found 

 combined with water in such proportions as to form a substance 

 chemically resembling opal, and of transparent glassy appearance. 

 Hence spicular sponges may be classified as calcareous or silicious 

 according to the nature of their skeletons. Spicules may have 

 one or two axes, or their axes may radiate in 3, 4, 5, 6, or even 

 8 different directions, and are found in a great variety of forms, 



