CORALLIFERI. 493 



combined action of its Polypi. This body is fleshy, and contracts or dilates in 

 its various parts by means of the fibrous layers that enter into its composition ; 

 its axis encloses a simple stony stem ; the Polypi have generally eight den- 

 tated arms. 



Most of the species diffuse a vivid phosphorescent light. 



Whatever be the general form of the Pennatulae, one of their extremities is 

 always destitute of Polypi, and has been compared to the tubular portion of a 

 bird's feather. 



PENNATULA, Cuvier. 



The Pennatula?, properly so called, have given their name to the whole 

 genus, which name has been derived from their own resemblance to a quill. 

 The portion destitute of Polypi is cylindrical, and terminates in an obtuse point. 

 The other part is furnished 'on each side with wings or laminae, more or less 

 long and broad, supported by spines or rigid setae which arise from their 

 interior and roughen one of their edges, without, however, being articulated 

 with the stony stem of the axis; it is from between their lamina? that the 

 Polypi protrude. 



In the fourth tribe the animal rind or bark encloses a mere fleshy substance 

 without an axis either osseous or horny. In 



ALCYONIUM, Linnaeus, 



As in the Pennatulse, we observe Polypi with eight denticulated arms, and 

 intestines prolonged into the common mass of the ovaries: but this mass is not 

 supported by an osseous axis; it is always fixed to the body; and where it is 

 drawn out into trunks and branches, nothing is found internally, but a 

 gelatinous substance traversed by numerous canals surrounded with fibrous 

 membranes. The bark is harder and excavated by cells into which the Polypi 

 withdraw more or less entirely. 



THETHYA, Lamarck, 



Where we observe the interior roughened with long, siliceous, spiral lines, 

 which unite on a similarly siliceous and central nucleus. The crust, as in 

 Spongia, presents two sorts of holes ; the first, closed by a sort of grating, must 

 be for the intromission of water, and the second, which are gaping, for 

 its exit. 



After the Alcyonia are also placed the 



SPONGIA, Linnaeus, 



Or Sponges ; marine, fibrous bodies, whose only sensible portion appears to be 

 a sort of tenuous gelatine which dries ofF, scarcely leaving a trace of it, and in 

 which neither Polypi nor other moving parts have yet been discovered. 

 Living sponges are said to exhibit a sort of tremulousness or contraction when 

 they are touched; it is also affirmed that the pores, with their superficies, are 

 perforated, and present a sort of palpitation ; the existence of these motions, 

 however, is doubted by Mr. Grant. 



Sponges assume innumerable shapes, each according to its species, and 

 resemble shrubs, horns, vases, tubes, globes, fans, &c. 



