112 
room xi. of a subcartilaginous fibre, strengthened by 
simple siliceous spicula. 
From them the Fresh-water Sponges ( Spon - 
gilla ) differ in being of a green colour. 
The Sea Oranges ( Tethyium ) differ from the 
Flinty Sponges, by being of a subglobular form, 
and harder, and by the spicula being placed in a 
radiating manner, and covered with a hard bark, 
formed of minute globular particles. These 
latter, when they are examined in a living state, 
may prove to be more nearly allied to the genus 
Cydonium . 
In Cases 19 and 20 are arranged the Coral - 
linesi which were formerly considered as animals, 
but which are now generally regarded as sea¬ 
weeds, containing a large quantity of calca¬ 
reous matter in their composition. Most of 
them are furnished with small tubercles similar 
to the fructification of Marine Confervae and 
other sea-weeds, and they are all destitute of any 
cells on their surface, which are always found in 
the corals. They have been divided into several 
genera, according to the structure of the stem. 
In most of the genera the stem and branches 
are articulated, the joint being united together 
by a central fibrous axis. 
In some of these, the branches are three¬ 
forked and fan-shaped, as the Corallina , where 
the joints are roundish, whilst they are flat¬ 
tened 
