466 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



variable in the extent of their development. One pair may extend nearly as far as the stomodreum, 

 while another may be merely incipient; further, the complete or incomplete pairs belonging to 

 opposite sides bear no bilateral relation to one another. 



In these genera, therefore, the mesenteries manifestly arise in single exocoelic pairs at almost 

 any region of the colony, though more freely in the legions of forward growth. The new 

 pairs, however, do not continue as a separate incomplete cycle, but become larger and larger, and 

 ultimately come into union with the stomodseum, while other new pairs appear in the meantime. 



Similar relationships of the mesenteries are also described for h<>j>li i/IHn, Furia, Af/nricin. 

 and others. In transverse sections mesenteries of all sizes are found, representing different 

 stages of growth, but without any regular alternation of small and large pairs; the Roman 

 numerals only approximately indicate any ordinal relationships of the pairs. Here again, one can 

 only assume that the different pairs arise for the most part independently of any cyclic plan, and 

 that each pair continues to increase in size, and may ultimately become complete. If the polyp 

 be in an actively growing condition, fission will again step in, the mesenteries which before 

 were incomplete now become complete, and new pairs continue to arise in the daughter polyps 

 in the same irregular fashion. 



When the very regular cyclic arrangement of young polyps of Mcmicina areolata is com- 

 pared with that after lission is well established (p. 503, et seq.), it is seen that the order of appear- 

 ance of the mesenteries is becoming fundamentally altered. It is manifest that single pairs arise 

 at any point, and grow independently of the others already present, so that in different primary 

 exocceles they may be one. two. three, or even four incomplete pairs. 



It may therefore be accepted as a general rule, that in genera reproducing by fission, the 

 mesenteries are not developed according to any regular cyclical sequence, once fission has become 

 established; but they arise as isolated exocoelic pairs, in regions of most forward growth, and 

 each and all the pairs may ultimately become complete. This is more fully illustrated under 

 fission in Manidna and Favia (p. 502j et seq). 



INCREASE OF MESENTERIES IX PORITES. 



As already mentioned, the tentacles and mesenteries in the genus Porites are always twelve 

 in number, and larval in the extent of their development, the Edwardsian mesenteries alone being 

 complete. Very exceptionally polyps are met with in which these organs may lie increased to 

 fourteen, sixteen, or even twenty-four, the polyps maintaining a circular form, like that of the 

 ordinary polyp, only larger. Similar numerical increases are likewise occasionally found in the 

 septa of individual corallites. A study of transverse sections of these larger polyps reveals that 

 the increase in the number of mesenteries proceeds in a manner different from any yet described 

 in the Madreporaria. The diagrammatic figures 10 and 11, and the camera drawings on PL V 

 (tigs. 41, 42), will serve to explain the various sequences followed. 



In tig. 41. and 10a, is represented a transverse section through the stomodwal region of a polyp 

 in which fourteen mesenteries are present, that is. two beyond the usual number. The twelve 

 primary mesenteries are easily determinable from the arrangement of the retractor muscles, and 

 retain their original condition, that is, four pairs (I-IV) are complete and two pairs (V, VI) are 

 incomplete. Within the sulcar or ventral entoccele, however, another complete pair (A. A) has 

 been added, and the sulcar directives are pushed farther apart. The retractor muscles on the 

 newly added pair are on the faces of the mesenteries turned toward each other, so that each forms 

 with the adjacent directive mesentery a unilateral pair, in which the retractor muscles are on the 

 mesenterial faces turned from one another as in directives proper. 



The next stage (lie. LOS) obtained is one in which eighteen mesenteries occur: fourteen are in 

 the same condition as in the previous polyp, and the four additional members are situated within 

 the entoccele of the seventh pair. The bilateral pair. B, B, is very rudimentary at this level, but 

 becomes proportionally better developed a short distance below the stomodseal region; each 

 member forms with the adjacent moiety of pair ( '. C a unilateral, anisocnemic pair, in which the 

 retractor muscles are vis-a-vis. A similar stage is represented in the next figure (tig. 10c), 

 except thai an unpaired complete mesentery is added within the entoccele of the last bilateral 



