100 Dr. J. E. Duerden on the 



While tins is taking place the fifth and sixth primary pairs 

 as a rule become complete. As already mentioned, Porites 

 among- corals is characterized by the mesenteries beyond the 

 protocnemic stage being added in bilateral pairs, disposed 

 pinnately within either the dorsal or ventral entocceles. 

 Further, the incomplete fifth and sixth pairs never become 

 united with the stomodaum. Figs. 2 and 3, with their 

 explanations, will make these differences clear. 



Diagram representing the order of appearance of the second cycle of 

 mesenteries in most modern corals. The primary mesenteries, /- VI, 

 are in the same condition as in the adult poh'ps oi Madrepora, except 

 that the filth and sixth pairs are nearly united with the stomodfeum. 

 Within each of the six primary exocoeles a new pair of mesenteries, 

 A-C, has appeared, the sequence being represented by the compara- 

 tive sizes of the mesenteries from the dorso-lateral to the ventro- 

 lateral exocoeles. Later all the primary mesenteries become inserted 

 on the stomodseum and the second cycle pairs are equal. Afterwards 

 the third cycle mesenteries arise successively in unilateral pairs 

 within the exocoeles between the pairs of the first and second cycle?, 

 first on one aspect of the second cycle mesenteries and then on the 

 other aspect, and afterwards become equal in size. Thus several 

 possible regions of growth occur within each prin.ary exocoele, each 

 capable of giving rise to an isocnemic mesenterial pair. 



Over a score of the large polyps of Madrepora were 

 sectionized transversely. All possessed more than twelve 

 mesenteries, and two stomod^al tubes, divided all the way, 

 were already present ; but as regards the mesenterial increase, 

 practically only three different stages are represented, in each 

 of which twenty-four mesenteries already occur at one phase 



