MADREPORARIAN CORALS 



35 



teries appears in the space between two directive mesenteries. 

 Thev appear in regular sequence (Fig. lo, A, B, C, D) in 

 bilateral pairs until the full number of twelve is reached. 

 Then the mouth and stomodaeum divide by constriction 

 as in the Astraeid, and when the polyp itself constricts two 



Figs. 



Fig. 9. Fig. iu. 



9 and 10. — Diagrams of sections of Poritcs to show the new set of 



Protocnemes is formed in space between the directive mesenteries III, III. In 

 Fig. 9 one pair, A, A, has been added, in Fig. 10 four pairs. After Ducrden. 



pairs of directive mesenteries are formed for each daughter 

 polyp by the mesenteries marked IV and / on one side and 

 III and a on the other, that is to say by one old directive 

 mesentery and one new one in each pair (Fig. ii). 



Thus it comes about that in these two genera, although 

 asexual reproduction is by fission 

 everv polvp in a large colony 

 has two pairs of directive mes- 

 enteries.^ 



The relation of the hard 

 calcareous septa of the coral 

 cup to the soft fleshy mesenteries 

 of the coral polyp which forms 

 it is a matter of considerable 

 importance for the proper under- 

 standing of coral anatomy. The 

 septa are alwavs formed in the spaces between the mesenteries 

 and never in the substance of a mesentery (Fig. 6, p. 32), 

 and, as the septa do not always correspond in number with 

 the mesenteries, the septa of the dried coral afford no trust- 



'■ J. E. Duerden, " West Indian IMadreporarian Polyps," Mem. Xat. 

 Acad. Sci. Washington, vol. viii., 1902. 



Fig. II. — Diagram to show a 

 stage in the division of a polyp of 

 Porites. Lettering as in Figs. 9 and 

 10. After Duerden. 



