ANATOMY OF MUSSA AND EUPHYLLIA. 35 



mesentery, Nos. 3 and 10 are those with filaments and are 

 longer than any others ; Nos. 1, 5, 8, and 12 are recognisable 

 for some distance below the stomodseum, Nos. 2, 4, 6, 7, 9, 11 

 are very short and rudimentary. In Pocillopora the difference 

 in size between 3 and 10 and the other longer mesenteries, 1, 

 5, 8, 12, is not so marked as in Seriatopora, and the last 

 named sometimes have rudimentary filaments. In both forms 

 the polyps show a well-marked bilateral symmetry with regard 

 to the dorsoventral axis, and in both there is a system of 

 superficial radiating canals by which the cavities of adjacent 

 polypes are put into communication with one another. 



In Madrepora Durvillei, according to Fowler (this 

 Journal, new ser., xxvii, pi. i), there is a well-marked dimorphism 

 in the polyps composing a colony. In the one form of polyp 

 there are twelve simple mesenteries, of which six have a long 

 course and a better developed filament than the remainder, and 

 of these two are longer than the others. In the second form 

 of polyp are also twelve mesenteries, which in the higher parts 

 of the polyp are perfectly normal, but at the lower end of the 

 stomodaeum six of them become modified, in all cases the same 

 six, namely, the 2nd, 4th, 6th, 7th, 9th, and 11th, using the same 

 method of counting as in Seriatopora. The modification consists 

 in the greatly increased size and vacuolation of the endoderm 

 cells of the mesentery, and in the presence of a canal lined by 

 endoderm, which runs right through its centre and is bent 

 sharply upon itself. For a description of the course of this 

 canal and its relations to the polyp cavity, Fowler's memoir 

 should be consulted. Only the modified mesenteries bear 

 filaments, and of them two, viz. four and nine, are longer than 

 the others. It is important to observe that the modified 

 mesenteries of the second type of polyp correspond with the 

 longer mesenteries of the first type, and that the longest 

 mesenteries of all occupy the same positions in both cases. 



In both Seriatopora and Madrepora Durvillei the two 

 longest mesenteries alone bear gonads. 



The differentiation of certain mesenteries in Madrepora and 

 Seriatopora, and the specialisation in both cases of two mesen- 



