ALCYONIUM. PENNATULA. 509 



by the siliceous character of the crystalline deposits, to which 

 they owe their firmness ; and, by some Naturalists, this species 

 has actually been referred to the class Porifera. 



1156. In one family of this Order, the polypidom is unat- 

 tached. Of this irroup, the Pennatula, or Sea-pen, is a charac- 

 teristic example. It consists of an axis which is stony for a 

 considerable part of its length, but is flexible at its two extrem- 

 ities. This is clothed with a flesh, which extends along the 

 sides into pen-like prolongations, arranged like the barbs of a 

 feather ; and one edge of each of these is fringed by Polypes. 

 The skin contains a large amount of calcareous spicula, and is 

 often deeply coloured ; being, in the British species Pennatula 

 phosphorea, purplish-red along the stalk, and orange at its ex- 

 tremities. It is not often that the Pennatula? rise to the surface. 

 They are usually brought up by fishing lines from considerable 



FIG. 717. FENNATULA. 



depths, where they live fixed in the mud by the lower extremity, 

 which is always destitute of pinnules and Polypes. It was for- 

 merly supposed that the Sea-pens floated freely in the water, and 

 that they were even capable of swimming by the action of their 

 fin-like lateral processes ; but there is no doubt that this notion 

 was erroneous, and that they are really rooted in the mud by the 

 basal prolongation of their stem. Some of the Pennatulee are 

 very brilliantly phosphorescent ; and the appearance of the 

 larger species, when displaying their luminosity in the dark 

 ocean, is very striking. The disengagement of light is, how- 

 ever, by no means constant ; but, as in all other instances of 

 phosphorescence, depends upon the condition of the animal. 



VOL. II. N N 



