544 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



Ill their typiciil. fully developed condition, the tentacles of the apical polyps are only six in 

 number (tig. Ih). They arise at the margin of the disk from the alternate entoca?lic spaces, and 

 may terminate either acutely or in a rounded manner; they are broadest at their origin, and a 

 wide interspace occurs b(>tween any two adjacent members. The surface is smooth throughout, 

 no urticating spots being visible. During full expansion the tentacles raaj* be overhanging and 

 digitiform; when only partly retracted they are seen as six short vertical processes, protruding 

 bej'ond the mouth of the corallite. The marginal spaces between the tentacles correspond with 

 the exoccelic chambers, and are sometimes rounded, while in other cases small processes, like 

 rudimentary tentacles, are present. 



The disk is circular, the central naked area being very small, and either flattened or slightly 

 convex, according to the state of distension. The twelve internal mesenteries, and their relations 

 to the other parts of the polyps, are easily seen through the transparent walls. The mouth is 

 extremely small, circular, and without prominent lips and gonidial grooves; sometimes the 

 lips are a little protruding. The diameter across the disk and tentacles is 6 mm., and the length 

 of a tentacle 2.5 mm. 



All stages, from the twelve tentacles present in radial polyps to the six characteristic of 

 typical axial polyps, can lie observed toward the margins of growing colonies; and a study of this 

 region of a colony, especially in the ])almate forms, indicates that almost any of the radial polyps 

 bj- excessive growth may l>ecome axial polyps. In doing this the polyp increases in size, and 

 at the same time its tentacles undergo moditication. The six entocoi^lic tentacles l)ecome still largei' 

 and all equal, while the six exocoelic become less important and in time wholly disappear, but for 

 a long time one or more of the exocrelic tentacles may be represented by mere processes. Thus 

 a true external dimorphism exists between the typical radial and axial polyps, though the one 

 m&y pass into the other. 



On several occasions the tentacles of an axial polyp were introverted, even while the column 

 wall and disk were still extruded; only six slight opacities remained to indicate the former 

 position of the outgrowths, and the margin of the column and disk as a whole was merely rounded. 

 The infolding of the tentacles was occasionally observed in the radial poh'ps; later, the tentacles 

 were slowly protruded, in the same way as described for the genus J'orittdi, where introversion 

 is more frequent and may be better observed. 



The radial polyps, even when expanded to their full degree, never protrude far lieyond 

 the aperture of the calice, and, owing to the oblique, usually iiariform aperture of the latter, the 

 amount varies in different parts of the same polyp. At the sides, where the wall of the corallite 

 is lowest, the colunm wall is free for 1 to 2 mm., but is not seen anteriorly. As the aperture of 

 the corallites is rarely directed upwaixl, th(> plane of the disk of the expanded polyps is oblique 

 to the axis of the colony, or may be directed inward to the sides, or in almost any direction. 

 Wherever exposed, the column wall is smooth, thin walled, and partly transparent. 



The tentacles of the radial polyps are, as a rule, twelve in number, but of diflerent dimen- 

 sions; six larger alternate with six smaller, the former communicating with the entocoeles 

 and the latter with the exocojles. The meml)ers of both series also vary in size among them- 

 selves. The larger tentacles are situated at the same distance from the center of the disk as the 

 smaller, so that practically the twelve constitute a single cycle, not differentiated into an inner 

 and outer cycle, as is most usual in coral polyps. All the tentacles are broadest at their 

 origin, but they narrow beyond, and may terminate bluntly or acutely. The relative sizes of 

 the different tentacles in each series have been alread}^ described, and are best undei-stood 

 from tigs, la-lo. 



The distinctly bilateral chai'acter which the tentacles give to the lateral polyps is most 

 marked toward the ends of the branches, where growth is rapid and the polyps larger. In partial 

 retraction the anterior tentacle usually protrudes beyond the margin of the corallite, and 

 considerably beyond the other mem))ers. In the more proximal regions of a colony, where the 

 polyps are, somewhat smaller, the tentacles tend to become approximately equal, but even here 

 the anterior tentacle can generally be recognized by its being slightly moi-c swollen, and lighter in 



