100 COMPARATIVE ANATOMY CHAP. 



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



axial skeleton of other dead Alci/onaria. 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 with 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, , 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 supporting 1 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 oesophageal 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 Corals, 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 Alcyonium, 

 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, within the Cnidaria is 



