MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 531 



constitutes the ventral directives, and the fourth the dorsal directives, while the fifth and sixth 

 jiairs, incomplete as _yet, arise on the ventral aspect of the second and first pairs, respectivelj-, the 

 fifth a little in advance of the sixth. (See diagrammatic figures on p. 508.) 



A young polyp, settled on the same hlock of dead coral as a mature colon}', aft'ords the next 

 stages required in the sequence — the manner of appearance of the first metacnemes. The living 

 characters are shown in fig. 109, and a section through the decalcified polyp is diagrammatically 

 repi'esented on p. 509. The protocnemes are in the same stage as in the previous figure — 

 the first four pairs are complete, but the fifth and sixth are still incomplete. Within the dorsal 

 and middle exocoeles on each side a pair of mesenteries has appeared, the dorsal pairs being 

 better developed than the middle. In fig. 15/", showing the arrangement in another decalcified 

 young polyp, six pairs of metacnemes have appeared, completing the second cycle, and all the 

 members of the first cj'cle are united with the stomodteum. The six metacnemic pairs thus follow 

 a dorso-ventral, or antero-posterior, order in their appearance, but are now practically equal and 

 constitute the second cycle of mesenteries. 



Fig. log, p. 510, shows the manner of appearance of the first pairs of mesenteries which will 

 constitute the third cycle of twelve mesenteries, or second cycle of metacnemes, and it is at this 

 stage that fission is introduced (p. 511). 



A tangential vei-tical section through one of the larvaj which had settled, but in which no 

 septal formation had yet taken place, is represented by fig. 110, and the right lialf of the same 

 section, more highly magnified, is represented by fig. 111. The four complete mesenteries extend 

 from the base to the upper wall, and present a muscular development on each face, the fibers 

 being cut obliquely. The endoderm is still greatly thickened, especially basally, while the 

 superficial ectoderm has undergone but little change; zooxanthelhc are altogether absent, 

 though present in abundance in the larva?. 



A great alteration has taken place in the basal ectoderm. It is no longer a broad columnar layer, 

 but is represented by little more than fragments, whi( h include a few nuclei and granular matter 

 which stains deeply. The mesoglcea is likewise extremely narrow except mesenterially, where 

 it is much broadened. In these regions can be seen structures similar to the wedge-shaped, 

 striated, desmoidal processes characteristic of adult poh'ps, so that evidently these arise at a very 

 early stage in the fixation of the larva; hints of the same processes also appear intermesenterially, 

 whei-e the mesoglcea is extremely narrow. The larva from which the sections were taken had 

 been adherent to a fragment of glass for over a week, and, though no septa were formed, it is 

 verj' probable that the basal plate had already been laid down, as this is one of the first parts 

 of the skeleton to appear. The skeletogenic ectodenu is in mucii the same condition as in 

 adult polyps, in regions where growth is not proceeding rapidh'. 



The passage from the narrow l)asal ectoderm to the broad ectoderm of the column at the 

 margin of the section is abrupt. The cells around the indented vertical part at the right 

 extremity of fig. Ill are somewhat modified compared with those beyond, and are probably 

 concerned in the formation of the epitheca. This is certainly th(>, case in the slightly older 

 polyp of Ilanicina represented in Hg. VM. 



YOUNG POLYPS OF MANICINA AREOLATA. 

 (PI. XIX, figs. 133-137.) 



To the verv complete description of the early stages in the development of this species given 

 by Dr. H. V. Wilson, in ISbiS, I have nothing to add, and will therefore proceed to the point at 

 which Wilson's i-esearches terminated, namel.y, the formation of the skeleton. The latest stage 

 reached in the growth of the Bahama specimens was one in which the twelve protocnemes 

 were present, only two pairs of which were connected with the stomod.eum. The stage is 

 comparable with that represented in fig. 13-1, PI. XIX {cf. Wilson's fig. 39), taken from a fixed 

 larva four days after extrusion from the jiarent colony. 



Out of many batches of larva^ extruded from a small Jamaican colony onlj' a few individuals 

 became fixed, and after several days these were reduced to two, which continued to live for nearly 

 three weeks, though under somewhat unfavorable conditions. The larva? were attached to frag- 



