30 



GUIDE TO THE COEAL GALLERY. 



Partition. 



Fig. 13. 



iigh Case IV. with a conspicuous rind {Gydonium japonicum, IV. 4), or rigid 

 E^igl^>. plates or masses of stony hardness {CoralUstes hoicerhanld, lY. 4). 



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



spicules, and the Litliistidii 

 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, liowever, 

 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- 

 iug 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 { Geo- 

 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 

 Order very minute S-shaped, 

 spiral, and stellate flesh-spi- 

 the affinities of the 



Trident spicules of Tetractiuellid Sponges. 

 (Magniiied 200 diameters.) 



cules, which are of great aid 

 various species. 



in 



determining 



