ANTHOZOA ASTEROIDA. 141 



and contraction of the polypidom renders it obvious that 

 the irritation has been communicated and felt through the 

 entire mass.* On the contrary, when at rest and undisturbed, 

 the polypes protrude their tentacula and a portion of the body, 

 and, imbibing the circumfluent water, this percolates into the 

 interior through numerous anastomosing canals, and distends 

 the polypidom so much that it will more than double or treble 

 its former size. In this respect the Anthozoa asteroida show 

 an affinity to the Helianthoida, and differ from the hydraform 

 and more especially from the ascidian orders. 



The axis of the Alcyonidse is imperfect, but exists neverthe- 

 less in the form of calcareous or siliceous spicula diffused 

 through the gelatinous interior, or more or less densely clus- 

 tered at the centre ; and the appearance of these spicula is 

 such that we are almost tempted to believe they may possibly 

 be the products of crystallization rather than of any regular 

 secretion. f It might not be difficult, but it is beyond my pro- 

 vince, to trace the gradual increase and consolidation of these 

 spicula through many intermediate species to the horny flexible 

 axis of Grorgonia, where it has become such an efficient sup- 

 port to the whole soft envelope as to claim not improperly the 

 name of its skeleton ; thence to the stony axis of the coral ; 

 and having there reached its maximum of development, I 

 might, on the other hand, have marked its progress towards 

 degeneration until it became again only a partial support, such 

 as we find it in the naked middle portion of the Pennatulidse, 

 more especially in some of the foreign and less typical species 

 of that family. 



* See " Reports" published by the Ray Society, p. 382. (1845.) 

 " Unknown to sex the pregnant oyster swells, 

 And coral-insects build their radiate cells ; 

 Parturient Sires caress their infant train, 

 And heaven-born Storge weaves the social chain : 

 Successive births her tender cares combine, 

 And soft affections live along the line." 



Darwin's Temp, of Nature, canto ii. 



t They may be compared with the Raphides found in the intercellular passages of 

 certain monocotyledonous plants. See Lindley's Introd. to Botany, p. 29. Mr. 

 Children found in the ashes of a piece of the axis of Gorgonia fiaJtellum a distinct 

 trace of pure silica, sufficient to form a globule before the blow-pipe. Ann. of Phi- 

 losophy, New Series, vol. ix. p. 431. 



