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CLASS IV. 
The semi-globular mass in the centre of the polypus 
may be looked upon as a collection of stomachs, 
separated by vertical partitions, or one stomach with 
eight divisions, having one mouth in common, sur- 
rounded by moveable appendages. But what may 
be the use of those tubercles on the projecting mem- 
brane found at the base, and surrounding the hemi- 
spheric mass, remains yet to be discovered. 
It is necessary to observe that the tentacula, the 
tubercles, the divisions of the hemispheric body, the 
intestines and the bands, always correspond in number. 
After this description, imperfect as it undoubtedly 
is, it will be easy to conceive how complicated is the 
organisation of the Alcyonian polypi ; many points of 
resemblance seem to unite them to the animals of 
the harder Polypidoms. 
These Polypidoms vary much in size ; some of 
them extend in patches of greater or less thickness on 
the surface of the bodies to which they adhere, whilst 
others raise their branches more than two metres high. 
Some live on marine plants ; others delight more 
in miry waters, and raise above the surface their ani- 
mated tubercles; the greater number attach them- 
selves to rocks and the larger shells, which they 
sometimes wholly cover with their polypean mass. 
They are found in all seas and at all depths ; never- 
theless, in consequence of the difficulty attending re- 
searches into the nature of these beings, the number 
of species known is as yet very inconsiderable, 
ARBORESCENT. 
1. Alcyonium arboreum . Stem tree-formed ; ex- 
