564 MEMOIKS OF THE NATIONAL ACADEMY OF SCIENCES. 



ORBICEI.LA ANNULARIS (Ellis &'Solander). 

 ( Pis. VIII-X figs. 64-7:;. ) 



External characters. — The species occurs on coral areas in small or large, tixod. nearly 

 spheroidal masses; also as an incrustation occupying areas several feet across. Small isolated 

 colonies are sometimes conical. In places it is an important constituent of the reefs. During 

 partial expansion the distal parts of the polyps are 2 or 3 millimeters apart, and on full 

 expansion the column becomes raised above the surface of the corallum for about ?. mm., in which 

 condition the individual polyps become so much enlarged that the columns may press against one 

 another. The column wall is smooth and cylindrical, and the proximal boundary of each polyp 

 is indicated by a polygonal shallow groove. When retracted, strong external ridges and deep 

 grooves correspond with the septa and mesenteries respectively. The costal ridges are alter- 

 nately large and small. The column can be partly drawn, iris-like, over the disk, but never so 

 as completely to hide it; on very strong retraction of the polyps the exsert edges of the cost a' 

 project greatly, as if perforating the tissue-. 



The tentacles are very short, appearing in two alternating cycles, usually of twelve each: the 

 inner and larger correspond with the primary and secondary septa, and the outer and smaller 

 with the tertiary septa. They vary very much in character under different conditions of expansion 

 of the polyp. Normally the tentacles are digitiform outgrowths, with a swollen apex, but during 

 lull expansion the walls of the polyps may be swollen to such a degree that the tentacular 

 walls become involved in the disk, and practically disappear, their position being indicated only 

 l>\ -lightly raised, light-colored, triangular areas, small and large alternating. In some polyps 

 the organs are imperfectly developed, so that no definite protuberances are represented; at 

 other times mere pyramidal processes appear at the apex of the costal ridges, a larger and inner 

 cycle alternating with a smaller and outer. During retraction the tentacles are hidden under 

 the overfolding column wall. The organs thus differ much from those in 0. radiata (p. 423). 



The naked portion of the disk is about 5 mm. across. It is deeply depressed when the polyps 

 are expanded, and almost completely hidden on retraction. The peristome maybe much or only 

 slighted elevated, but there are no true lips, the boundary of the disk and stomodseum being 

 sharply defined by differences in pigmentation. The mouth is generally open and oval shaped, 

 and stomoda?al ridges are not distinguishable. 



The color of the colonies as a whole is usually a steel gray or dark brown, and the tissues 

 generally appear dense and nontransparent. In some the tentacles and disk are a bright emerald 

 green, and an iridescent appearance is often produced by the presence of minute, white gran- 

 ules all over the column wall and middle region of the disk. The stomodseal wall is colorless. 



White, coiled mesenterial filaments may be extruded through temporary apertures in any 

 part of the superficial tissues, and may be again indrawn. 



Asexual reproduction takes place by intercalary and marginal gemmation. Buds with only 

 eight tentacles have been observed. 



Anatomy and histology. — The proximal region of the column wall is sinuous in transverse 

 sections, consisting of alternating larger and smaller ridges, separated by more or less acute 

 grooves; the former correspond with the costal evaginations, and the latter with the line of 

 attachment of the perithecal portion of the mesenteries. The wall consists of a broad ectoderm 

 and endoderm. but the mesoglcea is thin, except at the line of attachment of the mesenteries. 

 The outermost layer is provided with scattered nematocysts throughout its extent, and finely 

 granular, pigment matter occurs in its deeper regions, sometimes extending to the surface. The 

 granules appear as if irregularly distributed in patches among the various cellular constituents, 

 and in all probability they function as pigment matter, and give rise to the dense opaque appear- 

 ance of the living tissues; many clear gland cells occur in addition (rigs. 64, 65). 



Zooxanthellse are very numerous in the endoderm cells, and a layer of delicate muscle 

 fibrils can be made out in vertical sections, especially in the distal region of the column wall. 

 The layer is sufficiently well developed to warrant its being regarded as a special sphincter 

 muscle of the diffuse endodermal type (fig. »;."■). 



