POSTLARVAL DEVELOPMENT. 117 



159) separates the theca from the epitheca, as follows : " The essential differ- 

 ence which may be said to distinguish " theca " from " epitheca " is that the 

 theca, or wall, must be structurally associated with the peripheral ends of 

 septa, whereas the epitheca is in no structural connection with the septa, but 

 is a continuous concentric deposit exterior to all the other skeletal structures 

 of a calyx." And again (p. 248) : " The epitheca is an external basal struc- 

 ture, laid down at the angle of the aboral wall, where it bends towards the oral 

 or peristomal region of the polyp (figs. 22, 36). It is the continuation upwards 

 or outwards of the embryonic ' basal plate,' and may be well-developed or 

 remain rudimentary." G. von Koch (1896, p. 254) describes the structure 

 in much the same terms. 



These two definitions serve to distinguish clearly between the periph- 

 eral annulus found in Caryophyllia cyathus and that in S. radians. In 

 the one case it is structurally associated with the outer ends of the septa, 

 while in the other it is quite independent of these ; in one it is developed 

 within a special invagination, " thecal refoulement," of the skeletotrophic 

 wall which lines both its inner and outer surfaces, in the other it is wholly 

 external to the polyp, covered on one side only by the skeletogenic tissues, 

 not by an upfolding. In Caryophyllia von Koch found no trace of an 

 epitheca in addition to the theca, while in Astroides and now in the early 

 corallum of Siderastrea there is found to be an epitheca but no true theca. 

 Whatever calicinal wall is found in the mature corallum of these two genera 

 is a later structure formed by the coalescence of the outer edges of the septa 

 (pseudotheca). 



By Ogilvie (p. 248) and Vaughan (p. 48) the epitheca is regarded as a 

 primitive structure in Madreporaria, while the thecal structures are of 

 secondary origin. The latter writes : " The oldest type is where the ends 

 of the septa did not fuse distally, but simply had their outer ends bound 

 together by an epithecal covering." This clearly describes the early epithe- 

 cate corallum of Siderastrea. In this respect, therefore, the genus must be 

 considered as representing an older type than the truly thecate corallum of 

 such a form as Caryophyllia. Its very rudimentary condition in some of 

 the young polyps and its almost complete absence from the adult colony 

 would seem to prove that in this genus it is an embryonic structure of 

 diminishing importance. 



COLUMELLA. 



For a time the central part of the basal plate was free from any calcare- 

 ous deposit which could be regarded as a columella (plate 4), but, as the septa 

 increased in size, spinous upgrowths began to form near their inner extremity, 



