30 
GUIDE TO THE CORAL GALLERY. 
High Case IY. with a conspicuous rind ( Cydonium japonicum , IV. 4), or rigid 
Partition or masses of stony hardness ( Gorollistes Jjowerdanki, IV. 4). 
The Order is divided into two groups, the Ghoristida with separate 
fr IG> i 3 . spicules, and the Lithistida 
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 buudles. 
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 
Trident spicules of Tetractinellid Sponges. Order very minute S-shaped, 
(Magnified 200 diameters.) spiral, and stellate flesh-spi- 
cules, which are of great aid in determining the affinities of the 
various species. 
