502 MEMOIRS OF THE NATIONAL ACADEMY OE SCIENCES. 



The sulco-suIcHlar axis is a little to the side of the axial-abaxial plane. Only the eight 

 Edwardsian mesenteries bear mesenterial filaments, and these alone are complete. A pair of 

 metacnemes (A) have made their appearance in the exocoele on each side of the sulcar directives; 

 they are feebly developed, and extend for only a few sections below the termination of the 

 stomodifium. The three septa — two entosepta and one exoseptum — already developed at the 

 upper side of the bud are seen to be continuations of the costaj of the adjacent polyp. 



Fig. 62 is taken from a bud at a somewhat later stage, pi-eserved in a partly expanded con- 

 dition, so that sections could be obtained almost independently of the skeleton. The .specimen is 

 exceptional in that only five pairs of protocnemes are present, instead of the usual six. The 

 directives lie in the axial-abaxial plane, and development is most forward on the outer abaxial 

 aspect of the bud. Five alternating pairs of mesenteries, ])elonging to the second cycle, have 

 made their appearance within the primary exocceles, and the pairs exhibit a progressive order of 

 development from one aspect of the polyp to the other: the two lower pairs are the largest, the 

 musculature is well developed, and mesenterial tilaments occur at their free edge; the middle pairs 

 are smaller and without any trace of tilaments, while the uppermost pair is quite rudimentary. 

 Sections through five exsert septa are shown at the lower border, and are both entoccelic and 

 exoccelic. 



The section is of interest as indicating the tendencv to irregularities in the early formation 

 of the bud, but more particularly as exhibiting the pi-ogressive development of the metacnemes 

 and their filaments from one side of the polyp toward the opposite side, and the appearance 

 of both entoccelic and exoccelic septa in connection with the metacnemes, pari passu with the 

 growth of the latter. 



The section represented in fig. GU is through the protruding cone-like disk of a fully expanded 

 young polyp, and is of importance as showing that the fifth developmental pair of protocnemes 

 may become complete in advance of the sixth pair. On the left side is seen the outwardly 

 reflected lower edge of the stomodteum passing along the three lateral mesenteries, and the 

 ectoderm of the stomoda-um has assumed a very synmietrical form. Below the stomodfeum the six 

 pairs of protocnemes are ecjually developed, and the six pairs of the first cycle of metacnemes 

 have also made their appearance. 



Extratentacalar genuuation in Cl((docora proceeds therefore exactly as in Sohviastrwa, 

 except in the one important fact, the metacnemic succession is reversed. In the latter it is 

 from the dorsal to the ventral surface, while in the former it proceeds from the ventral to the 

 dorsal. However, before this exceptional sequence in Cladoconi can be regarded as established, 

 it will be necessary to confirm it on polyps arising directly from larvw. 



Transverse sections were made through a double polyp of ( 'ladoami, that is, one where two 

 oral apertures are surrounded by only a single system of tentacles and the two polyps have 

 only a common column wall. One polyp was normally hexamerous, having two orders of 

 mesenteries, not nuich difierentiated in size, and including two pairs of directives; it difl'ers in 

 no essential respect from a normal simple polyp. A small polyp to the left is evidently a bud 

 which has arisen on the discal wall of an older polyp, and as yet is imperfectly developed, 

 having only seven pairs of mesenteries, including one pair of directives. In the stomodwal 

 region four of the pairs are complete and two incomplete. 



If the sections lie compared with those of the fission polyps oi Mimiidrinii nudlutrid (p. 50.5, 



ct seq.) most important distinctions are at once manifest. In the two latter a normally hexamerous 



polyp has become divided through the stomoda^um into practically equal halves, each half 



having only a single pair of directives, while in Chulncord a new polyp is growing by the 



side of another, which still retains the primary arrangement of the mesenteries, including two 



pairs of directives." 



FISSION IN MANICINA. 



M(mictna areolata is one of the most favorable corals on which to study the process of 

 continuous fission, on account of the readiness with which small colonies provided with only a 



" The morphological significance of such double polyps in Cladocom and other corals has since been fully dis- 

 cussed in a paper on " Fissiparous Gemmation." See foot-note, ji. 496. 



