532 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



ments of glass, and could be transferred from one jar to another, or submitted to microscopic 

 examination. Usually they were kept in vessels in which living colonies of other corals, such 

 as ( 'ladocora and Oculina, were already established. 



Shortly after fixation the first four pairs of mesenteries reached the stomodseum, but the fifth 

 and sixth pairs remained incomplete for the whole period, and no trace of any metacnemes 

 appeared. The tentacles protruded toward the beginning- of the second week, and. at the time they 

 could be definitely recognized, were already twelve in number. 



One of the specimens, viewed as a transparent object, is represented in tig. 135. The meso- 

 glceal portion of the mesenteries appears as a clear, colorless line; the Edwardsian mesenteries 

 are united with the circular stomodseum, while the fifth and sixth bilateral pairs are incomplete. 

 The knobsof the retracted tentacles stand out as darker circular patches, arranged in two alternating 

 cycles of six each; the members of the inner cycle are entocoelic and those of the outer are exoecelie 

 in position, varying but little in size. Within the entocceles of each of the six pairs of mesen- 

 teries is seen the first indication of the skeleton, represented by narrow, septum-like deposits, 

 situated some distance from the periphery, and radiating toward the center. A basal granular 

 deposit, the first formation of the basal plate, could also be distinguished, but was not studied 

 in detail. 



The second polyp is represented in fig. 136, but the corallum has not developed to the same, 

 degree as in the first polyp. In the dorsal or sulcular entocoele the calcareous deposit forms 

 two small oval areas; in the sulcar entocoele the deposit is also oval and small; while in the 

 lateral entocceles it bears more i*esemblanee to a septum. The differences in extent of develop- 

 ment suggest that the six septa may not arise with complete uniformity, but under the unfavorable 

 conditions to which the polyps were subjected not much importance can be attached to the 

 result. Through the oral aperture two other skeletal deposits can be distinguished, the first 

 indications of the columella. 



Fig. 135 should be compared with that given by von Koch (1S97, p. 760) of the fully expanded 

 young polyp of Caryophyllia cyathus. Hen 1 , also, the corallum appeared at the Edwardsian 

 stage of mesenterial development; the tentacles are in two alternating cycles of six each, and 

 the six primary septa have appeared, but are more peripheral in their distribution and are 

 already united with the. circular theca. In Manicina no thecal formation occurred during the 

 short period the development was followed, but indications of an epitheca were observable. 



Both of the young polyps of Manicina were decalcified, with the object of ascertaining the 

 early stages in the modification of the polypal layers, consequent upon the formation of the 



skeleton. 



A radial vertical section through one of the polyps, including two septal invaginations, is 

 represented in fig. 137. It was from such sections that von Koch (1882) established the external 

 character of the skeleton in corals. The actual outlines of the polyp are from a camera lucida 

 drawing, while the diagrammatic outline of the skeleton has been added. 



The polyp is flattened in retraction, resting upon the skeletal upgrowths, and the mouth is 

 widely open. The right half of the section comprises a portion of a mesentery connected with the 

 stomodamm/and bearing a mesenterial filament; the left half includes the section of a tentacle, 

 which is only distinguishable from the rest of the ectoderm by its greater thickness and the pres- 

 ence of large nematocysts. The columnar and discal endoderm is narrow and contains many 

 zooxanthelhv, while the basal, skeletotrophic endoderm is greatly thickened, except over the 

 upper part of the septa. It is devoid of zooxanthelhe, and in its other characters closely recalls 

 the layer as it occurs in the lower part of the skeletotrophic tissues in adult polyps (tig. 129). 

 The mesoglua is extremely narrow throughout. 



Greatest interest attaches to the basal skeletogenic ectoderm. In the actual sections scarcely 

 any indications of the layer remain; it has either been removed by decalcification, or. more 

 probably, has become greatly reduced as a result of the formation of the skeleton, a condition 

 which has been found to characterize the older regions of most coral polyps. A few nuclei 

 occur here and there, and in places a detached mesogloea-likc membrane, representing the 

 skeletal membrane of Bourne. 



Where at each extremity the polyp turns upward the ectoderm for a short distance has 



