G. PLEXAURA. 
215 
The rind of the Plexauras is probably irritable, anti 
susceptible of contraction and dilatation, as the poly- 
pus can contract the borders of its habitation and 
wholly close it. In the species with large cells, such 
as the Plexaura friabilis, heteropora , &c. this cell al- 
ways remains open ; the polypus, instead of contract- 
ing its borders, shelters itself from the air and the 
contact of other bodies by horizontally extending a 
membrane, which forms the bag or peculiar envelope 
of its own body, but having an opening in the centre of 
this operculum or lid large enough to allow the water 
to penetrate to the bottom of its habitation. 
The axe of the Plexauras is not so smooth as that 
of the Gorgonias ; it is channelled, or rather wrinkled, 
and marked with points, which may be openings des- 
tined to maintain a communication between the inte- 
rior of the axe and the exterior envelope. 
These Polypidoms vary much in their form and 
size ; many are dichotomous ; some have their branches 
dispersed, and some almost pinnated ; there are some 
whose height is five centimetres, and others reach two 
metres ; in short, some are found of the thickness of a 
crow-quill, by the side of others whose branches are 
more than an inch in diameter ; and yet the number 
of species is by no means considerable. 
The Plexauras are not so brilliant in their colours 
as the Gorgonias ; usually they are of a dull reddish 
fawn, of various shades, which sometimes turns to 
an olive colour. 
They are found in the equatorial seas of both the 
New and the Old world, and also in the Mediterra- 
nean. From the magnitude of their cells and polypi. 
