POSTLARVAL DEVELOPMENT. 79 



mesenteries and septa in all these are never more than dicyclic. With the 

 later introduction of a radial polycyclic arrangement of the mesenteries and 

 septa, by the addition of exoccelic pairs in all the sextants, the fifth and sixth 

 protocnemic pairs begin to unite with the stomodseum, and so produce a 

 more approximate radial symmetry in the adult. But that the alternately 

 macrocnemic and microcnemic arrangement is more primitive seems to be 

 still suggested in the long separation from the stomodgeum of the fifth and 

 sixth protocnemes, found to be characteristic of recent actiniarian and 

 madreporarian polyps. 



The first twelve mesenteries in Siderastrea have been shown to arise in 

 bilateral pairs, a member on each side of the median axis, and the order is 

 such that each new pair appears alternately in the successively oldest 

 chamber. As the mesenteries become fully established another paired 

 arrangement is introduced. The directives throughout the life of the polyp 

 constitute bilateral pairs ; but now the second and fifth mesenteries on each 

 side form what I have termed unilateral pairs, and likewise the first and 

 sixth mesenteries on each side. Though varying so much in their order of 

 appearance and in their primary relations to one another, the six pairs in 

 the end constitute a regular cycle and appear all of the same value ; at first 

 they are anisocnemic pairs, but they become isocnemic when the fifth and 

 sixth bilateral pairs come into union with the stomodasum. Thus a truly 

 radial disposition of the parts results from a primary bilateral origin, a result 

 found to be continually recurring in the development of the different systems 

 of organs. 



SECOND CYCLE OF MESENTERIES (METACNEMES) . 



For a period of several weeks after fixation no addition to the twelve 

 protocnemes took place ; the four incomplete members continued to increase 

 in size, though remaining free from the stomodasum. Mesenterial filaments 

 began to show as dense, more opaque tissues within the interior of the polyp, 

 but their relative development could not be followed in the living polyp. 



Towards the end of the fourth week some of the polyps presented 

 rudiments of the second-cycle mesenteries. Their first appearance externally 

 was as two narrow lines along the column wall, towards its aboral termina- 

 tion, and within the dorsal exoccele of the right and left sides. A few days 

 afterwards these were followed by a similar pair of lines within each of the 

 two middle exocceles, and still later by a pair within each ventral exoccele 

 (plate 3, fig. 14). The actual time of appearance of the dorsal pairs, and 

 also the intervals between the three sets, varied somewhat in different polyps ; 



