RUGOSA ALCYONARIA. 175 



exceptional, and, on the, whole, the areas in which barrier reefs, encircling reefs, 

 and atolls occur were considered by him to be areas of subsidence. To this view 

 he was led in part by the difficulty of rinding any other foundation for atolls 

 than an island which had subsided. Direct indications of subsidence are, from 

 the nature of the case, difficult to obtain, but such evidence as we have is 

 not altogether in harmony with Darwin's theory. Thus Dana, although an 

 adherent of the view, was of the opinion that the movement now going on in 

 many of the Paumotu Islands is one of elevation ; and similar evidence is forth- 

 coming from elsewhere (Solomon Islands, Tonga Islands, etc.). Such evidence 

 is, on the other hand, in keeping with the view that atolls rest, at any rate in 

 some cases, on such banks as those described by Admiral Wharton, and referred 

 to above (p. 174). It has been urged by Murray that an atoll on such a basis 

 would increase in size by the extension of the reef to seaward on its own talus, 

 while the lagoon would be widened by the solution of the dead portions of the 

 reef. A fringing reef would be converted into a barrier reef by the same process. 



The part which the Antlwzoa take in the alteration of the earth's 

 surf ace 'is considerable. In the present time they protect the coast 

 from the consequences of the breaking of the waves, and assist in the 

 formation of islands and rocks by producing immense masses of 

 calcareous matter. In earlier geological epochs they have played 

 a still more important part, judging from the great thickness of the 

 coral formations of the Palaeozoic period and of the Jurassic formation. 



Order 1. RUGOSA = TETRACORALLA. 



Palaeozoic Corals with numerous symmetrically arranged septa 

 grouped in vmltiples of four. 



To these belong the families of the Cyathophyllidae, Stauridae, etc. 



Order 2. ALCYONARIA.* 



Polyps and polyp colonies with eight pinnate tentacles and eight 

 mesenteric folds. 



The Alcyonaria are all marine and, with the exception of the 

 Haimeidae, colonial. The buds are formed as a rule, not from the 

 bodies of the polyps themselves, but from stolons which originate as 

 tubular processes of the body-wall of the polyps at the base of the 

 colony (Fig. 145, A), or from the small canals which ramify in the 

 common jelly or coenenchyma and connect together the polyps 

 (Fig. 145, D). In the simplest colonies the polyps arise directly 

 from the basal stolon (Cormdarid) ; an advance upon this occurs 

 when the basal part of the polyp acquires a greatly developed jelly, 



* Wright and Studer, "Report on the Alcyonaria," Challenger Reports, 1889. 

 S. J. Hickson, " A Revision of the genera of the Alcyonaria Stolonifera," Trans. 

 Zool. Soc., vol. 13, 1894. A. Kolliker, Anat.-syst. Beschreibung d. Alcyonarien, 

 1872. 



