G. PLEXAURA. 215 



The rind of the Plexauras is probably irritable, and 

 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 friahilis, 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 wTinkled, 

 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. 



