MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 557 



tissues already noticed among the external characters. The coloration of the living polyp is 

 apparently superficial, for in no part of the ectoderm or endoderm can any elements, such as 

 pigment granules, be discovered, which may be considered as giving rise to the bright delicate 



colors. 



The ectoderm of the column wall is somewhat thickened at its lower termination, and passes 

 abruptly into the skeletogenic (basal) ectoderm lining the outside of the theca. Throughout the 

 polyp the mesoglosa is a perfectly clear, homogeneous layer, inmost cases indistinguishable from 

 the field of the microscope: an included cell occurs but rarely. In the distal region of t liv. 

 column the mesoglcea becomes broader, and immediately below the tentacular region is slightly 

 folded on its endodermal surface to afford additional support to the endodermal sphincter muscle. 

 The latter is hut feebly developed, and is continuous with the circular musculature of the 

 tentacles. 



The tentacles are both exoccelic and entocoelic in position. In sections through distended 

 specimens the ectoderm presents alternate swollen and narrow areas. The former correspond 

 with the tubercles noticed among the external features, and are constituted mainly of long, 

 narrow, thin-walled nematocysts with a close spiral thread, while the intermediate areas are 

 formed of a narrow epithelium resembling that of the column wall. In addition to the long 

 narrow nematocysts. the apical knob contains a few oval nematocysts with a loose spiral 

 thread; a few large granular gland cells also occur in the deeper regions of the apical and lateral 

 thickenings. In longitudinal sections through retracted tentacles the ectoderm appears as a very 

 thick, irregular layer, the nematocyst batteries overlying one another, as in the Solenastraea 

 (PI. X. tig. 7.">). The tentacular musculature is well developed, and immediately beyond is a layer 

 of very close, deeply-staining fibrils, with nuclei sparsely scattered among them. These extend 

 from the muscular layer for some distance, and end in a more or less distinct narrow zone. The 

 structure evidently corresponds with the nerve layer so frequently met with in the larger polyps 

 of the Actiniaria. 



In retracted tentacles the mesoglcea is folded on its endodermal border, and supports a 

 comparatively well-developed circular musculature. The endoderm is extremely narrow, the 

 cells are charged with protoplasmic contents, and show little or no vacuolization. Zooxanthellse, 

 such as occur in most Zoantharia, are absent, but here and there throughout the polyp are the 

 spheroidal, homogeneous, deeply-staining bodies, already described as occurring in Astrangia. 

 These are distributed more or less irregularly throughout the endoderm of the polyp, hut are not 

 so numerous as in the species just mentioned. 



The radial ectodermal musculature of the disk is arranged on tine mesoglceal plaitings, a 

 little stronger near the tentacular region; here also small nematocyst batteries are present. The 

 more central part of the discal wall is very delicate in all its three layers. 



The stomodseal tube is very short in retracted specimens, and its lower extremity is folded 

 backward and outward, and narrows inter mesenterially. The ectoderm is strongly ridged 

 vertically, the ridges corresponding in number and position with the insertion of the mesen- 

 teries; the mesoglcea and endoderm remain uniform all round. The ridges are practically 

 equidistant in transverse sections, and here the ectoderm hears large oval nematocysts and 

 granular gland cells; in the intervening grooves the layer is narrower, and constituted mainly 

 ..I' supporting cells. The filiation is uniform all round, and very delicate ectodermal and endo- 

 dermal muscular fibrils can he seen in sections. The ectoderm of the ridges terminates in direct 

 continuity with the filaments of the complete mesenteries. 



The mesenteries are hexamerous, hut the arrangement in alternating cycles is rarely 

 completed all the way round. In one polyp sectionized transversely (p. 4<i4). ten pairs of 

 mesenteries were united with the stomodseum, and of these two pairs were directives. The 

 ten pairs consist of six primary pairs and of four secondary pairs, the two remaining pairs 

 of this latter order being imperfect; in the uppermost stomodseal region, however, one of 

 the pairs becomes complete. An alternating tertiary order of twelve incomplete pairs may 

 occur, hut as a rule some pairs are rudimentary or absent. Thus there may he from six to 

 twelve complete pairs, according to the size of the polyp, and from two to twelve pairs of 

 Vol. 8— No. 7 11 



