326 CHARACTERS AND DIVISIONS OF ALCYONARIA. 



tures of a horny character, in general hardened by the deposition of 

 carbonate of lime. In some cases (e.g., in Cornularia) these horny 

 structures remain external, forming a sheath to the colony ; but in 

 other cases the colony becomes in process of growth inverted over 

 the skeleton, which then forms a central horny axis or " sclerobase " 



supporting the soft parts (fig. 210). 

 The mesoderm may also produce 

 skeletal structures in the form of 

 variously-shaped crystalline spicules 

 or "sclerites" (fig. 123), which are 

 composed of an organic basis har- 

 dened by carbonate of lime. In 

 some Alcyonarians the skeleton con- 

 sists solely of these mesodermal 

 spicules, which may remain en- 

 tirely free and detached (as in Al- 

 cyonium), or may be fused with one 

 another directly (as in Corallium 

 and Tubipora), or may be partly 

 free and partly united by horny 

 matter (as in Sclerogorgid). In 

 other types, again, as in Gorgonia 

 and Isis, the skeleton is twofold, 

 consisting on the one hand of an 

 axial sclerobase of ectodermal ori- 

 gin, and on the other hand of free 

 spicules developed within the mesoderm. The skeleton of Heliopora, 

 finally, is peculiar in its structure, and is probably ectodermal in 

 origin. 



The Alcyonaria, so far as the living types are concerned, are 

 divided by von Koch into the following groups : — 



Fig. 210. — Diagrammatic vertical section 

 through a young colony of Gorgonia, showing 

 the relation of the sclerobase to the soft parts. 

 «, Horny axis or sclerobase, supporting the 

 colony ; e, Ectoderm represented by the thick 

 dark line ; fi p, Two polypes ; c, One of the 

 ccenosarcal canals, connecting the visceral 

 chambers of the polypes with one another. 

 For the sake of clearness, the horny axis has 

 been represented of disproportionate thick- 

 ness. (After von Koch.) 



Sub-order I. Alcyonacea. — Sedentary Alcyonarians in which an 

 axial skeleton is usually wanting. When such a skeleton exists (as in 

 Corallium) it is of mesodermal origin, and is not secreted by a continuous 

 epithelial layer. 



Family 1. Haimeidae, 



. Monoxenia, Hartea. 



11 2. Cornularidae, 



Cornularia, dr^c. 



.. 3. Alcyonidae, . 



. Alcyonium, d^c. 



.1 4. Pseudaxonia, . 



. Corallium, Afopsea, <!W. 



11 5. Tubiporidae, . 



Tubipora. 



11 6. Helioporidae, . 



. Heliopora. 



Sub-order II. Gorgonacea. — Sedentary Alcyonarians in which there 

 is a firm sclerobasic axis secreted by a continuous epithelial layer. The 

 colony is not polymorphic. 



Family 7. Gorgonidae, 



Gorgonia, /sis, &■*<:. 



