474< NATURAL HISTORY OF AFRICA. 
and of geographical distribution. The number of 
species increase as we advance towards the warmer 
regions ; but to the north and the south of the lati- 
tudes just mentioned, they occur very rarely, and 
in but small quantity. 
Gorgonias, or sea-fans, millepores, corallines, 
sertularias, cellularias, alcyoniums, and sponges, 
are, in general, found most abundantly in the wa- 
ters of the ocean, in latitudes to the north and 
south of the continent of Africa. Yet, on the 
coast of Africa, as at Tunis, also in the Red Sea, 
the common officinal sponge, (spongia officinalis, 
im.) is fished up in considerable quantity, and 
forms a regular article of trade. 
