36 GILBERT 0. BOURNE. 



teries on opposite sides of the polyp for reproductive purposes, 

 suggests a close affinity between the two forms ; but there is 

 this difference between them, that whereas the reproductive 

 mesenteries in Madrepora Durvillei are 4 and 9, those in 

 Seriatopora are 3 and 10, whilst the short mesenteries of the 

 former correspond to the long mesenteries of the latter, and 

 vice versa. There are, however, other similarities in structure 

 which ally the Pocilloporidse with the Madreporinse, although 

 they are usually classed far apart, as Aporosa and Perforata 

 respectively. In Madrepora aspera and M. variabilis 

 the differentiation of the mesenteries does not appear to have 

 advanced so far as in M. Durvillei. In the first named, 

 according to Fowler, filaments are present on the abaxial 

 directive mesenteries as well as on the same six as in M. Dur- 

 villei, the remaining mesenteries being devoid of filaments. 

 Von Koch does not give any account of these structures in 

 M. variabilis. 



The Corallum. — In dealing with the hard tissues of 

 Madreporarian corals I shall try to point out that there are 

 only three (or possibly four) distinct structures in the corallum, 

 viz. the septa, the theca, and the epitheca (and perhaps the 

 basal plate). The columnella, the pali, the costae, the dissepi- 

 ments, synapticula, exotheca, and peritheca may all be traced 

 to modifications of the first named, and useful as they may be 

 for the determination of genera and species, they have no 

 morphological value. Associated with these names is the 

 view that some of these structures differ fundamentally from 

 the others ; some were considered by Milne-Edwards and 

 Haime to be of dermic origin, others to be of epithelial origin, 

 whereas, as we shall see in the sequel, all the hard parts are 

 of essentially similar origin. 



The first point of interest connected with the corallum is to 

 determine from which layer of the polyp it is derived, from the 

 ectoderm, endoderm, or as a calcification of the mesogloea. 



Milne-Edwards and Haime, in their classical work on corals, 

 stated that the calcareous tissue was deposited in a layer 

 which they called the dermis, which they defined as one of the 



