SOLITARY CORALS. 237 



cither side of instead of between the forked peripheral ends of the exosepta, and 

 these latter, as a consequence of the formation of new mesenterial pairs embracing 

 them, become in their turn endosepta. But, whichever the mode of peripheral 

 growth, it is obvious that the so-called " theca," about which so much discussion has 

 taken place, is really the result of the formation of radial structures in connection 

 with peripheral growth, and is not a circumferential structure. An examination of 

 the figure in the text, fig. IV., will make this clear. Here the different stages of 

 peripheral growth are clearly marked by the different distances from the centre of 

 the insertions of the primary, secondary, tertiary, and quaternary mesenteries. The 

 wall or theca must at each stage have been formed by the trabecular offsets of the 

 peripheral ends of the exosepta united to the endosepta, and it is clear that the 

 perforate " theca " of the adult is nothing more than a network of trabeculee formed 

 by the peripheral ends of the exosepta and endosepta. In Heterocyathus the wall is 

 formed by the intervention of the synapticula, which must be regarded as being 

 formed simply by very short radial folds between the outer ends of the septa, and are 

 therefore quasi-septal structures, as has been pointed out for Fungia by Miss Ogilvie. 

 In many corals the wall is formed by simple apposition of the thickened peripheral 

 ends of the septa, giving rise to the so-called " pseudotheca." In others it is formed 

 by the so-called "euthecal" pieces, which are really nothing more than very short 

 radial structures intervening between the peripheral ends of the septa, but not 

 produced internally so as to form septa. In the present state of our knowledge it 

 would be unprofitable to pursue the subject further, but I may repeat that the whole 

 tendency of recent investigation has been to show that a true theca, that is to say, a 

 circumferential structure independent of the radial growths, or septa, is always to be 

 referred to the prototheca, as defined by Bernard, and that all other so-called 

 "thecal" structures are in reality radial growths, formed in radial folds of the 

 polypal walls, between previously existing radial structures. Further, it would seem 

 from the most recent embryological investigations, which are confirmed by a study of 

 adult structure, that there are two ways in which the new radial growth may take 

 place : (1) by the simple formation of new radial folds between every pre-existing 

 radial fold (Caryophyllia, &c); (2) by the bifurcation of every alternate radial fold 

 and the formation of a new radial fold in every space formed by such bifurcation 

 (Siderastrcea, BalanophyUia, Heterocyathus, &c). 



It is possible, but it would be rash to make a positive assertion at present, that 

 these two modes of peripheral growth will be found to be of primary value to the 

 systematist. 



It may lie worth while, in conclusion, to observe that the following corals have 

 been found to have endosepta only : Mussa, Euphyllia, Madrepora durvillei, 

 Madracis aspenda, Duncania, Galaxea, the lateral polyps of Madrepora variabilis. 

 The following have been found to possess both exosepta and endosepta : Astroides, 

 Stephanotrochus, BalanophyUia, Hetei-opsammia, Dendrophylha, Heterocyathus, 



