554 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



toward its free edge, the color often disposed in lines. The general surface of the tentacles is 

 colorless, but green, white, or brown tubercles, and -an opaque white or green knob, are often 

 exhibited. The transparency and delicacy of the living tissues, as compared with other corals, 

 are due to the absence of zooxanthellse. 



On irritation the month opens suddenly, the polyps as a whole retract, and the column wall 

 becomes partly drawn over the disk and tentacles. 



Anatomy <nnl histology. — In the retracted condition nearly the whole of the column wall is 

 withdrawn within the calice; at most only a very narrow zone extends down the exterior of the 

 corallite. In sections the column wall is very narrow, becoming a little thicker at its proximal 

 termination. The ectoderm is seen to lie ciliated, and clear mucous glands are abundant, 

 surrounded by supporting cells. The mesoglcea appears only as a thin supporting lamella. A 

 feeble endodermal musculature can lie made out. and zooxanthellse are absent from the endoderm. 



The tentacles are both entoccelic and exoccelic in position. The ectodermal layer is broad, 

 especially at the tip, and contains long, narrow nematocysts, both terminally and in restricted 

 regions laterally. In retracted tentacles the nematocyst areas overlap the rest of the ectoderm; 

 nervous elements are clearly displayed at the tip of the tentacle, and gland cells with dear 

 contents are numerous proximally. The mesogbea is broader than in the column wall, and a 

 weak musculature is developed on either side. In the columnar endoderm. as elsewhere 

 throughout the polyp, occur numerous spheroidal bodies, usually with perfectly clear contents 

 and without a nucleus, and staining in carmine much more deeply than the other histological 

 elements of the tissues. In a few cases the spheroids are found with granular contents, or a 

 single nucleus may be detected; rarely others present what seem to be stages of reproduction 

 by simple fission. The bodies are of about the same dimensions and form as ordinary zooxan- 

 thelhe. hut their uniform structure, usually without any nucleus or vacuole, at once distinguishes 

 them from the symbiotic alga'. They present all the characters of nutritive unicellular organisms, 

 and similar bodies are not infrequent in other species of corals, and also in anemones. Though 

 occurring throughout the endodermal layer, they are most numerous in the swollen mesenterial 

 epithelium, generally considered to be the principal scat of intracellular digestion (tig. 4.")). Their 

 strong power of taking up coloring matter makes them very distinctive objects, and in any polyp 

 their distribution is somewhat irregular. 



In vertical sections of retracted polyps the central part of the disk is partly inturned, so 

 that it appears nearly in the same vertical line as the stomodaeal wall; but histologically the line 

 of separation is very marked and sudden. The ectoderm of the disk resembles that of the column 

 wall in the abundance of mucous cells, and displays a very weak ectodermal as well as an endo- 

 dermal musculature. The stomodseum is very short and partly reflected at the lower termina- 

 tion. Its ectoderm is constituted almost wholly of ciliated supporting cells, the aggregated 

 nuclei forming a distinct, deeply staining zone. The layer is uniform all round, no distinct ridges 

 and grooves being determinable as in the next species. 



Six pairs of complete mesenteries constitute the first cycle, and of these two pairs are 

 directives. In most cases two incomplete pairs occur within the primary exocoeles, instead of 

 one or three pairs, as the laws of Actinian symmetry require (tig. 47). Of the two pairs in 

 each system, one belongs to the second mesenterial cycle and the other to the third cycle, though 

 both are nearly of the same size. In one polyp sectionized transversely only six pairs of com- 

 plete and six pairs of incomplete mesenteries were present; but usually some of the tertiaries 

 occur in addition, though very rarely the complete cycle of twelve is represented. The order 

 of mesenterial increase, as revealed by the many polyps of a single group, has already been 

 described (p. 459, <t seq.). 



In the more distal region of the polyp the mesenterial musculature is well developed, 

 especially toward the insertion of the mesentery. The mesoglcea on the face bearing the 

 retractor muscle is thrown into complicated folds, varying greatly in degree in different mem- 

 bers. The opposite face of the mesoglcea may be also slightly folded, and the muscle fibrils in 

 places are strongly developed and appear vertical in direction. The musculature is nearly as 

 strongly developed on the secondary and tertiary mesenteries as on the primary. In the lower 



