544 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



In their typical, fully developed condition, the tentacles of the apical polyps are only six in 

 number (fig. 15). They arise at the margin of the disk from the alternate entoccelic spaces, and 

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

 wide interspace occurs between any two adjacent members. The surface is smooth throughout, 

 no indicating spots being visible. During full expansion the tentacles ma}* be overhanging and 

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

 beyond 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 Q 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 be observed toward the margins of growing colonies; and a study of this 

 region of a colony, especially in the palmate forms, indicates that almost any of the radial polyps 

 by excessive growth may become axial polyps. In doing this the polyp increases in size, and 

 at the same time its tentacles undergo modification. The six entoccelic tentacles become still larger 

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

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

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

 may 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 polyps; later, the tentacles 

 were slowly protruded, in the same way as described for the genus Porites, where introversion 

 is more frequent and may be better observed. 



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

 the aperture of the calice. and, owing to tin' oblique, usually nariform 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 column wall is free for L to 2 mm., but is not seen anteriorly. As the aperture of 

 the corallites is rarely directed upward, the plane of the disk of the expanded polyps is oblique 

 to the axis of the colony, or may he 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 different dimen- 

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

 and the latter with the exocceles. The members 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 already described, and are best understood 

 from tigs, la-l'i. 



The distinctly bilateral character 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 members. 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 more swollen, and lighter in 



