OF THE SPONGIADiE. 87 



most essential element, and in some genera, as in Spongia, 

 Spongionella, Verongia, and Auliskia, the skeleton is entirely 

 composed of it, in the form of anastomosing fibres. In 

 other genera the keratose fibres are strengthened either by 

 siliceous spicula or by grains of extraneous matter, selected 

 and incorporated in their structure by the fibres. In the 

 order Silicea it performs a much more subordinate part, 

 appearing only as a cementing material in the formation of 

 the various combinations of spicula of which the skeletons 

 of the sponges of this order are principally composed ; but 

 although in these cases only appearing as a subordinate 

 element, it is frequently very abundant. In the order 

 Calcarea it is less in the ascendant than in either of the 

 other orders, and in many species we are scarcely able to 

 distinguish it from the membranous tissues of the sponge. 

 Carbonate of lime, as an element of the skeleton, always 

 presents itself in the form of spicula of various forms in 

 combination with membranous structure. 



Silica in the skeletons of the order Silicea, presents itself 

 in a great variety of forms and combinations of spicula. 

 Sometimes the skeletons assume the shape of a beautiful 

 regular or irregular reticulation, composed either of a nearly 

 single series of elongate forms of spicula cemented firmly 

 together at their apices by keratode, or by numerous 

 spicula similarly cemented together, forming a strong and 

 complicated fasciculated thread of reticulations, as in the 

 Genera Halichondria and Isodictga. In other cases, as in 

 Tethea and Geodia, we find no reticulated structure, but 

 the spicula are arranged in elongated compound fasciculi, 

 which radiate from either the base or the central axis of the 

 sponge, and in Didgocylindrus we find the reticulate and 

 the radial system both entering into the structure of the 

 skeleton, a modification of the former prevailing in the 

 axis, and the latter existing in the peripheral portion of the 

 sponge. In Hymeniacidon neither of these modes of 

 structure exist, the spicula being simply and irregularly 

 dispersed over the membranous base of the skeleton ; and 

 finally we find it simulating the form of pure keratose 

 fibre, and becoming a rigid and solid siliceo-fibrous skeleton, 



