PROTOZOA: SPONGIDA. 



8 9 



siliceous sponges, both living and extinct, and the structure of 

 these is in many cases of the highest interest. Nothing further 

 however, be attempted here than to briefly characterise 



can 



I) 



Fig. 28. A, Dactylocalyx pumiceus, a Hexactinellid Sponge from the West Indies ; 

 B, A spicule of the Lithistid Sponge Discodermia, greatly enlarged, showing the 

 branched ends of the spicule ; C, Part of the skeleton of the Hexactinellid Farrea occa, 

 greatly enlarged, showing the continuous lattice-like framework, the component spi- 

 cules of which are only recognisable by their six-rayed axial canals ; D, Plan of a 

 single spicule of a Hexactinellid Sponge. (After Lutken, Sollas, and Carter.) 



the two principal groups of the Silidspongice viz., the Hexac- 

 tinellid and Lithistid sponges. 



i. HEXACTINELLID^:. In this group 'of the siliceous sponges the 

 skeleton is composed of six-armed spicules, the rays of which are almost 

 invariably at right angles to each other (fig. 28, D). In the centre of each 

 spicule are three canals, cutting each other at right angles and forming an 

 axial tube. The spicules become very commonly fused together by amor- 

 phous silica, so as to form a trellis -work of rectangular or polyhedral 

 meshes, the individual spicules of which are only recognisable by the 



