100 COMPARATIVE ANATOMY chap. 



of this Coral form a sort of crust over foreign bodies, preferring the 

 axial skeleton of other dead Alcycmaria. The surface by which they 

 adhere to these bodies, and which is of course ectodermal, puts forth 

 externally, and thus between itself and the surface to which it adheres, 

 a lamella of horn, which, together ivith the foreign body (axial skeleton 

 of another Alcyonid), forms the axial skeleton of the whole stock. 

 " Now, however, it not infrequently happens that the Gerardia colony 

 tries later to spread out further than is allowed by the surface it rests 

 on, and then growths which bear young polyps appear on its branches, 

 and into these a new formation of horn enters, attached to the 

 original horny secretion ; this new formation has a similar origin 

 with the first, but encloses no foreign body " (v. Koch). In the other 

 Alcyonaria which have a horny axial skeleton, the formation of the 

 skeleton is similar (Fig. 82, B, p. 107), but that part of the axial 

 skeleton which is attached to a foreign substance is very much reduced, 

 whereas the free part rising from it is considerably developed, and 

 forms the generally much-branched principal mass of the skeleton. 

 Horny axial skeletons are thus always lined with an ectodermal axial 

 epithelium. In the axial skeleton of the Alcyonaria, lime may be 

 found in larger or smaller quantities, and sometimes preponderates. 

 In Isis horny joints alternate with calcareous. The mesoderm of the 

 soft part of the Coral stocks which covers the axial skeleton often 

 contains calcareous spicules. In such cases {e.g. Gorgonia) we have 

 an ectodermal horny axial skeleton and a more peripheral mesodermal 

 skeleton formed of calcareous spicules. 



2. Mesodermal suppopting organs. — The first of such organs 

 which we shall consider is the thin structureless membrane, which, 

 throughout the whole Hydroid body, separates the ectoderm from the 

 endoderm. In the Craspedote Medusce this membrane thickens into the 

 more or less strongly developed structureless elastic disc jelly ; it is 

 retained as a thin membrane only in the tentacles, and generally also 

 in the cesophageal tube. In the Scyphomedusce the mesodermal sup- 

 porting jelly begins to be more highly differentiated, connective tissue 

 cells appearing in it, and fibres, which are either processes of such cells 

 or differentiations of the intercellular substance (Fig. 36, p. 41). In 

 the same way we find, in the Cm-als, a hyaline mesodermal layer 

 throughout which cells are scattered. The membrane is everywhere 

 reduced to a thinner layer in the tentacles. In many Corals (most 

 Alcyonidce) the skeletal elements are found in this layer. They con- 

 sist of variously shaped calcareous spicules, which arise in special cells 

 and are found in varying numbers. In the mesoderm of Alcyoniwn, 

 and in the peripheral portions (the rind) of the stock of the precious 

 Corals of commerce and other forms, they are isolated. Occasionally, 

 however, a new calcareous substance is deposited between the calcareous 

 spicules, cementing these into a firm mass, and leading to the formation 

 of axial skeletons such as that of the precious Coral of commerce. 



The greatest differentiation, histologically, vnthin the Cnidaria i& 



