42 6 MEMOIRS OF THE NATIONAL ACADExMY OF SCIENCES. 



maturity they migrate to the periphery, and arrange themselves in a vertical direction, parallel 

 with the other cellular constituents. 



A weak longitudinal ectodermal musculature seems to be always present, the cut ends of 

 the fibrils being displayed in transverse sections, and most pronounced toward the proximal 

 extremity. In some species — e. g., Cladocora, Madrepora, and probably others — a distinct 

 nerve layer also occurs, situated some distance from the mesoglcea (PI. II. rig. IQnr. I.). The 

 ectodermal gland cells and supporting cells are practically the same as in the column wall, 

 but the former are less numerous. 



The tentacular mesoglcea is always a very thin layer, usually smooth on both surfaces, while 

 the endoderm is comparatively broad with irregular internal limitations. The endoderm is 

 generally richly supplied with zooxanthellse, but the alga? are absent from Phyllangia, Astrangia, 

 and certain of the tentacles of Madrepora. In all instances a weak circular musculature is 

 developed, but the mesoglcea is rarely folded to afford it additional support, as in the larger 

 tentacles of Actinians. The lumen is preserved, even in fully retracted tentacles. 



The tentacles of several genera present so many peculiarities of form and arrangement as to 

 call for special description. 



TENTACLES OF MADREPORA AND PORITES. 



The tentacles of the polyps of Madrepora and Pontes are exceptional among the genera 

 studied in that they are, with certain exceptions, only six or twelve in number, and in the 

 adult usually exhibit constant variations in size. The tentacles of the apical polyps of 

 Madrepora will be first described (rig. 1. a. I>). In the most typical instances only six tentacles 

 occur, all equal in size, and communicating with the entocoelic chambers. They are widest at 

 their origin in the margin of the disk, where a considerable interval separates one from another, 

 and terminate either acutely or in a rounded manner. The surface is smooth throughout, no 

 urticating spots being visible. 



Polyps with such a tentacular system are found at the apex of the long established branches of 

 colonies of both the palmate and arborescent types of growth. On polyps at the ends of short, 

 rapidly growing branches, rudiments of other tentacles also occur, alternating with the members 

 of the first order. In regions of vigorous growth, as at the margin of palmate colonies, it is 

 found that certain of the ordinary polyps, bearing the full complement of twelve tentacles, may 

 become larger and assume an apical character, and among these the separation of the tentacles 

 into an inner ami an outer cycle can be recognized. Sometimes, only two or four of the six 

 members of the outer cycle will be present, always much smaller than the entotentacles. All 

 stages in the diminution in number and size of the outer exoccelic tentacles are. however, 

 represented, according as the polyp has recently assumed or long maintained the apical position; 

 at the same time, the six members of the inner cycle become larger and more equal. 



From all the variations observed, it is clear that on any polyp taking on the axial condition 

 tlie six exoccelic tentacles, present on all the radial polyps and smaller from the beginning, tend 

 to completely disappear, and only the six entocoelic members ultimately remain, becoming at the 

 same time larger and equal. Like the other regions of the axial polyp, the tentacles are perfectly 

 colorless, owing to the absence of zooxanthellae, ami are not often seen fully expanded. 



Among the fully developed radial polyps of Madrt pora twelve tentacles occur: rarely, the 

 number may be increased to sixteen, eighteen, or as many as twenty-four. The usual forms and 

 arrangement are given on PI. I. tig. 1 (<].-)>.). The separation, as regards distance from the center 

 of disk, into two alternating cycles of six each, is not clearly defined, but the members of one 

 series are always larger than those of the other. The anterior or abaxial tentacle, adjacent to 

 the nariform apex of the corallite, is longer and stouter than any of the others, and colorless, 

 except toward its origin. It may be nearly twice as long as the others, and stands out very 

 prominently; even in partly retracted polyps, when the tentacles are arranged vertically, it 

 easily overtops the rest(c). The opposite or axial tentacle— that is, the one adjacent to the stem- 

 is the next in size, but differs very little, sometimes not at all, from the four large lateral 



