30 



GUIDE TO THE CORAL GALLERY. 



High Case 



Right 



Partition. 



IV. with a conspicuous riud {Cydonium japoniciwi, IV. 4), or rigid 

 plates or masses of stony hardness {GoralUstes hotverhanki, IV. 4). 

 The Order is divided into two groups, the Ghoristida with separate 



spicules, and the Litlnstida 

 with peculiar " desma " spi- 

 cules, which are usually articu- 

 lated to form a rigid stony 

 skeleton. Some species possess 

 " caltrop " spicules, with four 

 axes and four rays. The most 

 characteristic spicule, however, 

 is the trident, with a long 

 shaft and three prongs, which 

 may project forwards or be 

 bent backwards or outwards 

 (Fig. 13). The tridents are 

 arranged with the shafts point- 

 ing inwards and the prongs 

 spreading tangentially beneath 

 the surface or projecting out- 

 wards. Tridents and needles 

 in varying proportions often 

 form thick radiating bundles. 



The Geodine Sponges (G^^-o- 



dia, Cydonium) possess a thick 



outer crust or rind, composed 



of solid globular spicules 



(Fig. 14). The "desmas" of 



Lithistid Sponges are formed 



by the deposition of concentric 



layers of silica round a minute 



rod or caltrop ; on this nucleus 



there arise nodulated branches, 



which articulate with the 



branches of other desmas to 



form a rigid framework. In 



addition to the " skeleton " 



spicules, there occur in this 



TrideutspicuiesorTetractineliid Sponges. Order very minute S-shaped, 



(Maguifitd 200 diameters.) spiral, and stellate flesh-spi- 



cules, which are of great aid in determining the affinities of the 



various species. 



