ACTINOZOA. 



251 



other cases there is present, in addition, a secondary calcareous mem- 

 brane, which is known as the " epitheca" and which appears to be 

 secreted by the reflection of the ectoderm over the upper part of the 

 corallum. The "dissepiments" which are present in the interseptal 

 loculi of most corals, are secreted by the ectodermal cells ("calico- 

 blasts ") of what were " originally the interseptal parts of the base of the 

 polype'' (Bourne). The soft parts of the polype thus always lie above 

 the last-formed dissepiments, and the spaces between the latter and the 

 theca are not occupied by soft tissues of any kind. 



In connection with the preceding, a few words may be said as to the 

 relations of the polype to the corallum which it secretes. As has been 



-cy 



Fig. 131. — Diagrammatic vertical section of Mussa corymbosa, showing the relations of the 

 polype to the corallum. od. Oral disc of the polype ; cc, CEsophagus, with the free edges of the 

 mesenteries seen below its inferior end; mn, Flat surface of a mesentery; ec, Ectoderm (cross- 

 shaded); en, Endoderm (unshaded), the dark line between this and the ectoderm representing the 

 mesoderm ; ca, Special ectodermal layer (" calicoblast layer ") secreting the corallum (co) ; d, Dis- 

 sepiments; rr, Inversion of the polype constituting the "Randplatte." (Slightly altered from 

 Bourne.) 



seen, the corallum is of ectodermal origin, and is therefore really external 

 to the polype. In process of growth, however, the corallum becomes 

 pushed up into the polype from below, so to speak, and thus comes to be 

 apparently situated within the polype. What actually happens, as shown 

 in the accompanying diagram (fig. 131), is that the polype becomes in- 

 verted over the corallum, so as to form a layer externa/ to the theca, 

 which von Heider has spoken of as the " Randplatte." Thus a portion 



