MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



511 



The polyps of Favia growing directly from larvae are thus seen to follow a very definite 

 sequence in their mesenterial growth, a sequence which hears the closest resemblance to that 

 characteristic of other larval corals which have been examined. Each is provided with two 

 pairs of directives, and the mesenteries are arranged in two or three radial cycles, according to 

 the usual hexameral plan. It is at about the stage represented by fig. I5g that fission is introduced. 



Two larval polyps were secured, each provided with two oral apertures, hut still sur- 

 rounded hv only one system of tentacles and a simple column wall. They thus represent the 

 earliest stage in fission. At such a stage the polyps are very short, and rather irregular in form 

 when preserved, so that it is practically impossible to secure in one section the complete arrange- 

 ment of the mesenteries around the stomodseum. The diagrammatic figures are therefore con- 

 structed by combining the relationships of the mesenteries in the various serial sections. In each 

 polyp the stoiiiod;vum has been divided throughout its length into two distinct tubes, and a 

 definite number of mesenteries is associated with each. 



The mesenterial system of one of the double polyps is represented by tig. 16//. Compared 

 with the stage in fig. 15g two additional pairs of mesenteries have become complete, so that four 

 pairs are inserted on each stomodseum. Instead of the two new complete pairs arising as a pair 

 on each side of the directive plane, as considerations of symmetry would suggest, both are 

 situated on one side of the angulated directive axis of the polyp, though from the section alone 



Pig. 16A. 

 Favia fragum. — First stage of tission in a larval polyp. 



it is impossible to say which of the four pairs actually represent the additions. The members 

 of the second cycle include only a single pair of mesenteries in each exoccele, except in the two 

 exocoeles adjacent to the right pair of directives, where third-cycle pairs are developed. 



The two polypal halves are thus nearly alike, the original single stomodseum having been 

 divided practically clown its middle, so that half the mesenteries are attached to each moiety. 

 The plane of fission crosses the directive axis, passing through the entoccele of two lateral pairs 

 of complete mesenteries on opposite sides. Growth is taking place more vapidly at the upper 

 right side than elsewhere, and the pair of directives has thus been pushed to one side of the 

 median plane, so that the directive plane no longer divides the polyps into equal halves. 



Were the halves to he completely separated at thisstage.it is clear that a pair of mesen- 

 teries would be formed in each new polyp, by the approximation of a mesentery from the two 

 opposite sides, the musculature in the two moieties being on the faces turned toward each 

 other; an ordinary pair of mesenteries would be thus produced, and each polyp would have 

 but one pair of directives. Thus, from the beginning, an important difference in the nature of 

 the mesenteries would be established between fission polyps and single polyps reared directly 

 from Iarvse; the distinction between the orders to which the mesenteries primarily belonged 

 also begins to be lost. 



