40 GILBEET 0. BOURNE. 



septa usually project higher into the body cavity than does their 

 product the theca, and the extrathecal mesenteric chambers, 

 (exocceles and entocoeles of Fowler) are continuous with the 

 corresponding intrathecal chambers over the lip of the calyx. 

 I have attempted to show these complicated relations in the 

 diagram fig. 14. Fig. 15 exhibits diagrammatically von Koch's 

 view of the formation of the corallum (without epitheca). 



It is worthy of remark that, according to von Koch's obser- 

 vations, the basal plate in corals has a different developmental 

 history from the theca, and is morphologically distinct from it. 

 As far as I am aware, no one has yet called attention to this 

 fact. The basal plate, however, is, according to the same author, 

 continuous, if not identical with the structure known as 

 epitheca. 



The epitheca is formed in Astroides as a secretion of the 

 ectoderm of the body wall at a spot where the lateral walls of 

 the polyp pass into the basal portion ; it is connected with the 

 basal plates and form a thin and tolerably smooth lamella in- 

 vesting the lower parts of the polyp (vide fig. 13, ep.~). 



Unfortunately, the further development of the epitheca has 

 not been studied, and we are even deficient of an exact know- 

 ledge of its structure in the adult. In his most recent contri- 

 bution to the subject (' Morph. Jahrb./xii, 1886, p. 154) von 

 Koch has given a diagram which professes to show clearly all 

 the relations between soft and hard tissues in the adult coral 

 polyp. In it the epitheca is figured as a complete and inde- 

 pendent wall of calcareous tissue lying parallel with the theca, 

 and separated from it by a considerable space of extrathecal 

 coelenteron, this space being bridged over at intervals by the 

 costse, which in the drawing abut upon and are fused with the 

 epitheca so as to connect it with the remainder of the corallum. 

 Such relations as are shown in this figure occur, as far as I am 

 aware, in no coral either living or extinct; they can only be 

 considered as theoretical, and form a part of the system which 

 von Koch is attempting to construct on the subject of the 

 coral skeleton. I have never seen or heard of a coral in which 

 the soft tissues outside the upper part of the theca are them- 



