MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 419 



whether this ever occurs naturally, as sections reveal only a great thinnino- of the layers. Polyps 

 of Madrej>ora, Cladocora., and Astrangia, having a tul)ular calic(% are able to withdraw their 

 upper parts so deeply within the latter as to render the disk and tentacles almost invisil)le. 

 Most members of the Astneidic also partly withdraw themselves within the calice, and at the 

 same time, by the contraction of the sphincter muscle, the capitular region of the cokimn wall 

 is drawn, iris-like, over the disk and tentacles, leaving but a small central aperture through 

 which the mouth and central part of the disk can usuallj' be seen. In Ifanicma and ColpoplnjUla 

 the columnar musculature is weakly developed, and when retracted the capitular region is 

 partlv drawn downwardly and inwardly, covering the tentacles, but leaving the middle discal area 

 exposed. In Madrcpora, J\)rltes, Siderasfrpea, and Agaricla the column wall is verj^ rarely 

 overfolded; on retraction the disk and tentacles are merely drawn downward, coming to rest 

 upon the corallum, and the tentacles, disk, and mouth remain exposed. 



During full expansion the upper part of the polyp is elevated some distance beyond the 

 corallum, and the perithecal portion of the gastro-co?lomic cavity becomes swollen." The 

 column wall, instead of being folded horizontally or downward over the tbeca, now stretches nearly 

 vertically from its line of union with the other jwlyps and skeleton as far as the tentacular zone. 

 This alteration of form can ha easilj^ understood in the case of distinct polyps, but not so 

 readily in species where the polyps are incompletely separated. 



Manicina is a good example of the latter in which to compare the different appearances of 

 polyps on expansion and retraction. In the latter condition the meandering disk rests upon the 

 skeletal projections on the floor of the calice, and the upper part of the columnar expansion lies 

 obliquely upon the upper edge of the septa, then folds over the margin of the theca, and is con- 

 tinued downward over the outside of the theca for a distance varying in different examples. On 

 full distension the disk is raised several millimeters above the skeleton, becoming much 

 broader and flattened, or even convex; the tentacles are arranged in a marginal zone, either 

 overhanging or partly involved in the discal tissues. The column wall is elevated vertically, 

 its lower margin being the line along which the superflcial tissues pass into the tissues lining 

 the skeleton, and this for the time being constitutes the lower fixed termination of the column. 



Jfpeandrina and (_'olj)oj>li>jlli(( are somewhat more complicated. The living colon}' during 

 the day usually exhibits a meandering system of columnar ridges and discal vallej-s; the column 

 extends about half way within the calice, folded and slightly swollen as it terminates, and more 

 or less hiding the rows of tentacles. Full distension completely reverses the relationship of the 

 disk and column wall. The former now becomes raised from its depressed condition along the 

 floor of the calice until it is some millimeters wholly above the corallum, and convex in vertical 

 section; the adjacent colunui walls are also raised until they ])econie nearly vertical, and are 

 either pressing against one another laterally', or separated only by a deep, narrow groove, at the 

 bottom of which lies the line of connection of the column wall to the skeleton. The former discal 

 valleys are now the ridges, and the thecal ridges the bottom of the valleys.* 



A few observations have been made with regard to the external conditions which seem to 

 determine the state of expansion or retraction of coral polyps. As a general i-ule the polyps are 

 not expanded to their full degree during the day, either on the reef or in the laboratory; but 

 the process begins immediately after sunset, and full expansion is maintained for the greater 

 part of the night. Thus on bringing into the laboratory, in the course of the morning, a collection 

 of specimens, they usually remain retracted for the rest of the day, 1)ut after sunset (6.3(1 to 7.15 

 p. m. in Jamaica) the polyps begin to expand until they attain their full dimensions. The 

 bodv cavitv is greatlv distended with water, and the column wall and disk become raised some 



"Where the pericalicular continuation of the gastro-ccelomic cavity has become broken up into irregular canals, 

 as in Mudrcpora, the amount of distension is small; but even in this genus a marked difference is seen iti the c«>no- 

 sarc, according as the canals are fully charged or nearly empty. 



'' Verrill (1863, p. .38), from an examination of alcoholic specimens of Maaudrina, Mniiieitui, and Ftirbi, came to 

 the conclusion that the polypal disk does not rise even level with the summit of the corallum. Also naturalists 

 familiar with the Bahama and Bermuda cnrals have informed me that they ha\e never seen many nf the fissiparous 

 species (e. g., hophyllin) in an expanded state. 



