MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 571 



The polyps arc often expanded during the day. Imt like most other corals are seen at their 

 best at night time. The superficial tissues arc then raised for several millimeters above the 

 skeleton, and appear thinner walled and slightly more transparent, allowing the white skeleton 

 to be seen through, and the various stages of fission to be more readily followed. White 

 mesenterial filaments can be extruded through any part of the polypal wall. 



Colonies have been found at various times of the year bearing tree larva?. Occasionally 

 simple young polyps are met with attached to older colonies, evidently derived from larva? 

 which settled directly in the neighborhood of the parent. The hexameral cyclic plan prevails 

 throughout the .simple polyps, but is lost when fissiparity commences. 



Anatomy <ni<] histology. — The ectoderm of the column wall is a narrow layer with numerous 

 clear eland cells, but no granular eland cells, nematocysts, nor muscle fibrils are distinguishable. 

 The mesogJoea is nearly as broad as the ectoderm, and connective tissue cells are present here 

 and there. The endoderm is crowded with zooxanthellse, and delicate circular muscle fibrils 

 can be made out. 



The tentacles are very short in sections of partly retracted polyps. The ectoderm of the 

 swollen apex is a broad layer, its peripheral half crowded with long narrow nematocysts. bearing 

 a closely spiral thread. In the deeper regions occur numerous, strongly stained examples in 

 different stages of development; a larger nematocyst, showing the central axis very distinctly, 

 may occasionally be seen. The urticating spots noticed amongst the external characters appear 

 in sections as lateral rounded elevations of the ectodermal layer, and bear nematocysts similar to 

 those of the knob. The merest trace of an ectodermal musculature occurs. In preserved 

 specimens the tentacular endoderm practically leaves no lumen above, its cells being largely 

 vacuolated and crowded with zooxanthellse; the muscular fibrils are also slightly better developed 

 than in the ectoderm. The discal endoderm consists mainly of clear gland cells, and the ectodermal 

 and endodermal musculatures are of the weakest character. 



The stomoda'um is very short in vertical sections, terminating below in a rounded edge 

 intermesenteriallv. while mesenterially it is continued into the mesenterial filament. Where the 

 stomodseum is widely open the surface is even, but where it is closed strong vertical ridges and 

 furrows occur. The ectoderm is a very uniform layer of strongly ciliated supporting cells, with 

 interspersed large oval nematocysts, and the endoderm is nearly devoid of zooxanthellse. 



The number and arrangement of the mesenteries are very variable, depending upon the size 

 of the polyp and the rate of growth at any particular region. Three orders can usually be made 

 out, though rarely presenting a cyclic regularity (fig. 93); no directives have been found in 

 the many polyps sectionized, though they occur in the larval polyps. The last cycle of 

 mesenteries extends but a short distance vertically, and all the mesenteries may bear filaments. 

 The mesogloea is usually strongly developed, and one face is much plaited for affording support 

 to the retractor muscle. Toward their lower termination many of the mesenteries are convoluted 

 at their filamental edge, but the foldings are not so numerous as to crowd the gastro-ccelomic 

 cavity. 



The mesenterial filaments are of the usual type, with the swollen mesenterial epithelium behind 

 highly vacuolated. Some of the filaments become greatly enlarged proximally, and charged with 

 large oval nematocysts with the thread arranged in a wide spiral; transverse sections through 

 these are not infrequent in sections, and present a very characteristic appearance (tig. !"4). 

 Two or three of the filaments in each polyp undergo a special glandular modification (p. 474); 

 for a short distance vertically all the cells, with the exception of a few supporting cells, become 

 enlarged and charged with a bright yellow, finely granular substance. 



Only entoccelic septal invaginations occur throughout the greater part of the polyp, alter- 

 nately large and small, corresponding with the similar pairs of mesenteries (fig. 93). In the 

 uppermost region, however, exocoelic invaginations extend a very short distance inwardly and 

 vertically. 



Proximally. the interseptal loculi are never wholly cut off from one another, there being no 

 -olid columella in the corallum. 



