REPORT ON THE ALCYONARIA. IX 



the mesoderm of the body-wall of the polyp the skeletal elements may be greatly 

 developed, so as to constitute the former a stiffened cylinder. The first beginning of 

 such is to be seen in some species of Telesto, where the spicules of the body-wall so 

 interlock as to produce a marked rigidness. Such a support is, however, well developed 

 in Tubipom, where the spicules of the mesoderm coalesce with one another, so as to 

 transform both the mesoderm of the polyps and of the stolons into solid tul)es. In 

 Heliopora, finally, we see crystalline calcareous masses appearing in the abundantly 

 developed ccenenchyma, and the colony becomes a porous coral mass. 



Another series of progressive modifications may be traced on the separate individuals 

 of the various colonies, which have for their object the protection of the soft parts of the 

 polyps, especially the oral region and the tentacles. These modifications are not 

 unimportant as characterising the main divisions, and are of much use in determining the 

 mutual relations of some of the families and genera. In the simplest cases, when the 

 polyp contracts, its oral region is invaginated, and it folds the tentacles over the 

 oral aperture. If the tentacles are on their abactinal surface furnished with spicules, they 

 act in virtue of their position as a certain protection against external assault. Such 

 conditions are exhibited by the species of several families and genera ; thus among the 

 Alcyonacea in Cornulariidse {Rhizoxerda and Anthelia); among the Scleraxonia in many 

 Briareidse ; among the Holaxonia in the Dasygorgidas. A progressive step in this adapta- 

 tion of the tentacles for protective purposes is exhibited by many polyps, in which under 

 the bases of their tentacles long spine-like spicules have developed. These surround the 

 tentacles and oral region as a circle of sharp projecting spines, as in several species of 

 Spongodes, Ceratoisis, and Acanthogorgia. If these spicules be broad triangular discs 

 which may be laid down over the infolded tentacles, opercula are formed which close the 

 entrance to the soft parts, as in the Primnoidse. At the same time the body-wall of 

 the polyp always forms by means of embedded calcareous spicules a more or less rigid 

 structure. 



In Muriceidse the polyps exhibit a still more diff"erentiated structure. In these each 

 polyp consists of a basal portion thickly beset with spicules, which projects from the 

 ccenenchyma of the calyx, then of an oesophageal region which is soft and without spicules, 

 and lastly of an oral region including the oral disc and the tentacles, which at their bases 

 are surrounded by a collaret of spicules. The bases of the tentacles are on their abactinal 

 surfaces thickly beset with spicules. The entire oesophageal portion may be invaginated 

 within the calyx, and over this the armed tentacles form an operculum. In its expanded 

 condition the polyp is able to bend its body in diverse ways, since no spicules hinder the 

 free mobility of its oesophageal portion. 



A still greater freedom of motion is attained when the base' of the polyp being 

 rigid, the rest of the body remains soft and without, or with very few, spicules. The 

 flaccid body can then be completely invaginated within the calyx, and closed by the 



