ANATOMY OP MUSSA AND EUPHYLLIA. 41 



selves invested by a calcareous lamina. In all the forms with 

 a persistent epitheca which I have examined it is invariably in 

 the form of a more or less well-defined layer of calcareous 

 tissue, investing the basal parts of the corallum, and. nowhere 

 extending above the level of the soft parts covering the exte- 

 rior of the latter. Its relations and general appearance suggest 

 its having been formed from the free edge of the soft tissues on 

 the exterior of the corallum, as they retreat farther and farther 

 from the original surface of attachment. 



The names exotheca, peritheca, ccenenchyme, epitheca, are 

 all applied to laminar, ring-shaped, or encrusting calcareous 

 investments of the theca, and the distinctions drawn between 

 them are so subtle or so vaguely expressed that I am quite 

 unable to distinguish the difference between them in ordinary 

 cases. In point of fact no essential difference exists. If these 

 structures are deposited by the same parts of the polyp in each 

 case they are morphologically similar to one another ; variations 

 of form count for nothing. To determine the mode of forma- 

 tion of corallum, when embryological data are not to hand, a 

 study of the microscopical characters of the corallum by means 

 of sections, and a consideration of the relation of the soft parts 

 to the corallum is necessary. In the first place, it must be 

 kept in mind that throughout the region of the living polyp 

 the corallum is invested by an active secreting layer of calico- 

 blasts. Fowler's figures of Lophohelia, and mine of Mussa, 

 Euphyliia, and Astrsea, show that there are present in the 

 septa dark lines or centres of calcification marking the central 

 point from which calcification has taken place. In Mussa it 

 can be easily seen that concentric layers have been formed 

 around the centre in each septum (fig. 2), and that as two 

 contiguous septa became joined together the area of active 

 secretion was confined to the external or peripheral part of the 

 septum, which therefore increased in length centrifugally. A 

 section taken somewhat lower down than in fig. 2 shows that 

 calcareous tissue is also added over the bridges connecting the 

 septa until they are connected together by a very solid theca. 

 Compare with this section the diagrammatic figure of a coral 



