viii 
Gorgonia, whose axe is inarticulated. The third Or- 
der contains the Isideae, whose polypi are more ap- 
parent, and the axe articulated. 
The fourth Class has no Order ; it is denominated 
Carnoid Polypidoms, composed of a fleshy mass, 
wholly animated, covered with polypi, and possess- 
ing no central axe. 
SITUATION. 
The greater number of Polypidoms are found to 
originate in the heart of the ocean, and in its nume- 
rous divisions, with the exception of the Naissas and 
the Ephydatias, which always inhabit fresh water, 
whether stagnant or current. 
Some of the Polypidoms appear to prefer the im- 
mediate influence of the atmospheric changes ; they 
are seen on the rocks and on the plants which the tide 
leaves uncovered, and sometimes in such profusion, 
that all appear concealed under an animated mem- 
brano-calcareous covering. These species, however, 
at least on our shores, are neither numerous, nor at- 
tain any considerable size. 
They almost all suffer from the action of the air. 
At the period of the great equinoctial tides, the sea 
retires from those rocks it covered for many prece- 
