12 
Tank No. 21. 
The chalky sponges ( Calcarea ) are mostly small, and either grey or white; 
a fair type is the Sycon raphanus of which a variety peculiar to the 
Aquarium grows thickly on the walls of the tanks (Fig. 1). The flinty 
sponges [Silicea) are the most numerous and varied; 
to these belong in the Aquarium the orange-co¬ 
loured branches of Axinella (Fig. 2), the yellow 
balls of Tethya (Fig. 3); also the apparently free- 
moving Suberites (tank 23, see p. 44), under 
Fig. 1. Some specimens of 
Sycon raphanus, attached 
to a piece of rock at the 
left hand. 
Fig. 3. Tethya lyncurium, 
on a stone. 
Fig. 2. Axinella , on a stone, V 2 hat. size. 
which, however, will be found in such case a crab using it as a protec¬ 
tive covering. 
The sponge of commerce is of the form we know in its domestic 
