. The annals and magazine of natural history : zoology, botany, and geology. aterialcollected later it appears that when six new pairs of mesenteries havebeen formed the enlarged polyp, now possessing twenty-four mesenteries,may undergo division iuto equal halves, at any rate so far as concerns theoral aperture, stomodseum, and mesenteries. The mesenteries in eachdaughter polyp are arranged exactly as in ordinary polyps arising asbuds. The introduction of fission, however, in no way niodities themorphological significance of the pinnate order in which the increase ofmesenteries takes place. 10
. The annals and magazine of natural history : zoology, botany, and geology. aterialcollected later it appears that when six new pairs of mesenteries havebeen formed the enlarged polyp, now possessing twenty-four mesenteries,may undergo division iuto equal halves, at any rate so far as concerns theoral aperture, stomodseum, and mesenteries. The mesenteries in eachdaughter polyp are arranged exactly as in ordinary polyps arising asbuds. The introduction of fission, however, in no way niodities themorphological significance of the pinnate order in which the increase ofmesenteries takes place. 100 Dr. J. E. Duerden on the While tins is taking place the fifth and sixth primary pairsas a rule become complete. As already mentioned, Poritesamong- corals is characterized by the mesenteries beyond theprotocnemic stage being added in bilateral pairs, disposedpinnately within either the dorsal or ventral , the incomplete fifth and sixth pairs never becomeunited with the stomodaum. Figs. 2 and 3, with theirexplanations, will make these differences Diagram representing the order of appearance of the second cycle ofmesenteries in most modern corals. The primary mesenteries, /- VI,are in the same condition as in the adult pohps oi Madrepora, exceptthat the filth and sixth pairs are nearly united with the 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 insertedon the stomodseum and the second cycle pairs are equal. Afterwardsthe third cycle mesenteries arise successively in unilateral pairswithin the exocoeles between the pairs of the first and second cycle?,first on one aspect of the second cycle mesenteries and then on theother aspect, and afterwards become equal in size. Thus severalpossible regions of growth occur within each
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