MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 4">7 



pairs, the two larger arc situated within the dorsal or sulcular exocoele on cadi side of the polyp, 

 and the two smaller pairs are within the middle exocoele on each side (see also, PI. XIV. fig. L09). 



In addition to these two instances, a number of early mesenterial stages have I a secured 



from young buds, and there is little doubt thai the process of metacnemic development in these is 

 exactly the same as that followed by polyps with direct larval predecessors. 



Fie\ 86, on PL XII. represents a transverse section through the stomodseal region of an 

 expanded bud of Solenastrsea. The protocnemes are in the Edwardsia-stage of development, 

 and within each of the dorsal or sulcular exocceles a pair of metacnemes has appeared. Sections 

 of the same bud, taken a little below the inner termination of the stomodsaum, reveal, in addition, 

 a rudimentary pair of metacnemes within the middle exocoele on the right side, though no trace 

 of new mesenteries appears in the left middle exocoele; the sulcular pairs are also much further 

 developed than in the upper region, and are much larger than the single middle pair (fig. 87). 



The stomodseal region of another hud of Solenastrsea, somewhat younger than the former, is 

 represented on PL XI, tig. 82. Of the protocnemes, four pairs again arc complete and two 

 pairs incomplete. In this instance the first two pairs of metacnemes are very rudimentary, and 

 appear within the middle lateral exocceles, not as before, within the sulcular exocceles. The new 

 pairs are slightly better developed in sections somewhat lower, but no dorsal or ventral pairs were 

 encountered. 



In later buds of Solt mist run six pairs of metacnemes occur, a pair within each of the primary 

 exocceles, and these exhibit a developmental succession from the sulcular (dorsal, axial) aspect to 

 the sulcar (ventral, abaxial). In older buds all the pairs are equal in size, forming a regular 

 hexameral second cycle, and this is the adult condition of most of the polyps in a colony (tig. 81). 



The diagrammatic representation of the mesenteries of a young polyp of Astrangia solitaria 

 on [>. 460, also indicates a like dorso-ventral succession for the second order of mesenteries in 

 this species. 



Buds of < 'ladocora arbuscula reveal somewhat similar conditions in the appearance of the 

 metacnemes (p. 45S). Fig. 61, on PI. VIII. represents a transverse section through a bud in 

 which two pairs of mesenteries are present, in addition to the six pairs of protocnemes; of the 

 latter only the Edwardsian mesenteries are complete and bear mesenterial filaments. The two 

 pairs of metacnemes (A. A) are very rudimentary, and extend but a short distance down the 

 column wall, and are devoid of mesenterial filaments. The proportional development of the 

 protocnemes enables the dorsal and ventral aspects of the polyps to be determined, and serves 

 to indicate that in Cladocora the first metacnemes appear within the ventral or sulcar exocoeles, 

 as compared with their dorsal or sulcular origin in Soli nastrasa (PI. XII). Comparing fig. 87 with 

 fig. 61, the sulcar aspect in both genera is seen to he the outer or abaxial with regard to the rest 

 of the colony, while the sulcular is the inner or axial; therefore, in the two species the metacnemic 

 succession proceeds from opposite aspects. 



PL VIII, fig. 62, represents a transverse section through a somew'hat older hud of Cladocora. 

 The specimen is exceptional in that the dorso-lateral pair of protocnemes is missing from the 

 left side, so that the polyp is pentamerous. Five alternating pairs of metacnemes also are 

 present, and their interest in the present connection lies in the fact that they show a marked 

 gradation in tin 1 extent of their development, in passing from the outer to the inner aspect. 

 Mesenterial filaments occur on the pairs in the sulco-lateral exocoeles, and the longitudinal 

 muscular fibers are also determinable. This latter character is apparent on the metacnemes in 

 the middle lateral exocoeles. but no trace of mesenterial filaments occurs. The single pair in the 

 sulculo-lateral exocoele is very rudimentary. The proportional development indicates the same 

 relationship as fig. 61. namely, that the metacnemic sequence is from the abaxial to the axial 

 border of the polyp. At a little later stage the polyps of Cladocora consist of six protocnemic 

 pairs, all the members complete, and of six alternating metacnemic pairs, all incomplete and 

 equally developed. Many polyps in a colony are found in this condition. 



The manner of appearance of the first cycle of metacnemes in asexually produced coral 

 polyps is thus in closest agreement with that in larval polyps. In both cases they arise as 

 isocnemic pairs within the six primary exocoeles, and in bilateral order from one aspect of 



