CORALS AND CORAL-ANIMALS. 195 



light-liver to a bright golden-brown ; exceptionally it is a light stone-3cllo\v, greenish-brown, 

 bright-bronze- or verdigris-green. It has been observed of this type, by the author, that the 

 individual coralla are capable of changing their characteristic superficial colour within a short 

 interval of time. A specimen kept in a basin of constantly-changed sea-water was, when originally- 

 collected, of a bright verdigris-green hue ; a day later it had assumed a commoner and more 

 sombre olive-brown ; but a few days subsequent!}' it reverted to its original verdigris tint. It 

 would appear, from the phenomenon now recorded, that the collective polj'p stock exerts some 

 specific control over the superficial pigment corpuscles of the common corallum. 



The colour and aspect of the associated polyps of Sarcophyton glaucum present little varia- 

 tion. In their condition of full extension, these polyps stand out from the fleshy disk on slender 

 stalks, that vary from half-an-inch to nearly an inch in length. The eight-rayed tentacular star 

 that surmounts each slender stalk is almost invariably bright lemon-yellow ; while the supporting 

 stalk itself, most commonly dark liver-brown, may be a lighter shade of the same tint. The 

 extended polyps of this Alcyonarian are extremely minute, not exceeding one quarter of an 

 inch in diameter; and, being crowded together in close proximity, countless myriads are included 

 in a single corallum of the ordinary size. Such is the close order in which these polyps are 

 associated, that no part of the corallum is visible when they are all fully extended. 

 Representations of the living coralla of this species, together with the polyps in various 

 stages of extension, are included in Figs. 18 to 20 of Chromo plate No. X. As there 

 shown, the tentacular lobes are not iringed after the manner of those of the majority of 

 Alcyonarian polyps. 



In a second form, identified with the Alcyoniitin latum of Dana, but probably referable 

 to the genus Sarcophyton, the corallum is represented by fleshy folia, deeply indented on one 

 side and radiating from a common centre. Its colour is liver- or golden-brown, and the 

 extended polyps are bright lemon-yellow with brown stalks, as in the type last described. A 

 typical illustration of this species is presented in the foreground of the upper reef-view in Plate 

 No. XX. A third type, identified by the author with the Alcyoniitin niuralc of Dana, builds up a 

 corallum resembling a corrugated sheet of light liver-coloured leather, that may cover an area of 

 many yards. A single extensive colony-stock of this variety, as illustrated by the lower reef-scape 

 in Plate XX., may, in point of fact, occupy the greater portion of the field of view. The polyps 

 of this species, also, as in the preceding types, are primrose-yellow with brown stalks. A large 

 portion of the reef-scape of Plate XIX. is occupied by a species, apparently identical with the 

 Alcyonium flexile of Dana, in which the edges are developed into attenuate digitiform prolongations. 

 Both the polyps and the supporting stalks in this species are primrose-yellow. 



It is characteristic of the species of the two genera Alcyonium and Sarcophyton, just 



described, that the polyps in their retracted state are withdrawn entirely within the substance 



of their polyparies. In another section of the Alc^'onarian order, the polyps, when 



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