80 
ROOM XI. 
Nat. Hist. 
every direction, while in the calcareous sponges the body 
is harder, rather elastic, composed of subcartilaginous 
fibres, and supported by stellate, calcareous spicula. 
The Flinty Sponges are rigid and friable, composed of 
a subcartilaginous fibre, strengthened by simple silice¬ 
ous spicula. From these the Fresh-water Sponges 
(Spongilla) 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 glo¬ 
bular particles. These latter, when they are examined 
in a living state, may prove to be more nearly allied to 
the genus Cydonium. 
In Case 19 are arranged the Corallines, which were 
formerly considered as animals, but are now generally 
regarded as sea-weeds, containing a large quantity of 
calcareous matter in their composition. Most of them are 
furnished with small tubercles similar to the fructification 
of Marine Confervae, and are 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 cen¬ 
tral 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 flattened in Flabellaria. In 
Jania the branches are simply forked and slender, with 
long cylindrical joints. In Amphiroa the branches 
are forked, and the joints compressed, and separated 
by longer spaces. In others, the stems are membrana¬ 
ceous, tubular, and covered with a chalky coat. Of 
these, some are furcately branched, as the Galaxura , 
where the branches are jointed, whilst in Dichotomaria 
they are continuous. In Penicillus the forked branches 
arise from the end of a cylindrical stem, which is fur¬ 
nished below with a fibrous root. The genus Poly - 
physa 
