MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 417 



better developed, and symbiotic algse or zooxanthellse are nearly always present. The citation is 

 feeble, and rarely determinable in preserved material. 



The endodermal layer is of much the same character throughout the polyp, whether in the 

 column wall, tentacles, disk, skeletotrophic tissues, or forming the mesenterial epithelium. It 

 may vary slightly in thickness in different regions, and in the greater <>r less preponderance of 

 glandular cells, while in nearly all the species a remarkable modification of the skeletotrophic 

 endoderm takes place in the lower regions of the polyp. The layer here becomes much thicker 

 and loses its distinctly cellular character, appearing finely reticular. So greatly thickened does 

 the endoderm become that it often nearly obliterates the gastro-coelomic cavity in the most 

 proximal region of the polyp. The chief constituents — nuclei, cytoplasm, zooxanthellse, and in 

 some cases granular gland cells- are mostly accumulated, in a narrow peripheral zone, the deeper 

 portion being vacuolated or bearing only tine granules (PI. X, figs. 73 and 75). 



Zooxanthellse occur in large numbers within the endoderm cells of all the species studied. 

 with the exception of Phyllangia americana and Astrangia solitaria. They are usually 

 distributed throughout the polyp, but are more numerous in the exposed tissues (column wall, 

 disk, tentacles) than in the endoderm of the mesenteries and skeletotrophic tissues: they 

 even occur within the internal canals of the perforate genera Mddrepora and Pontes, but are 

 never found free or detached within the polvpal cavities except in larvae. As described on 

 page 437, the organisms are the principal cause of the coloration of many coral polyps. Large 

 oval nematocysts occur in the endoderm of Pm i it<K and Madrepora, but are absent from most 

 other genera. Their numbers and distinctive form in the genera mentioned are such as to leave 

 no doubt that they are actually formed in the endoderm, not free examples injested from the 

 ectoderm. 



The circular endodermal musculature of the column wall appears to be always present in 

 coral polyps, as in Actinian polyps, though varying much in the degree of its development; 

 as a rule it is stronger at the uppermost region of the column wall than below. Sometimes 

 the fibrils are scarcely to be found anywhere, while in other species they become strongly 

 developed distally. and give rise to a typical diffuse sphincter muscle, such as is characteristic 

 of many Actinia' (e.g., Corynactis). This is seen in species of Orbicella, especially in the 

 large 0. cavernosa, but also in the smaller 0. annularis (PI. VIII, fig. 65). Here, in retracted 

 polyps, the mesogloea is thrown into deep folds for additional support to the musculature. The 

 muscle fibers lining the hollows or grooves never become separated from the superficial layer, 

 as happens in Actinians where the muscle is truly mesoglceal. In other species of corals 

 the mesoglcea forms only very slight folds, while again it may be perfectly smooth, indicating 

 a very weak muscular development. 



The sphincter muscle is more strongly developed in Isophyllia dipsacea than in any other 

 species here studied. In vertical sections of the uppermost region of the column wall the mesoglcea 

 displays one or more special thickenings which are much plaited, the whole lined with muscle fibers 

 (PI. XVII, fig. 121). The structure very closely recalls the type of sphincter described by Iladdon 

 (1898, p. -ir'rj) as occurring in the Actinian Macrodactyla, and there termed a '"restricted" 

 sphincter muscle. It represents a stage of muscular development more complex than that 

 described as "diffuse." The plaitings appear on several axes of greater or less complexity; while 

 in the "circumscribed" sphincter muscle of Actinian anatomy they are restricted to a single axis. 

 The amount of development of the sphincter muscle is manifestly dependent upon the size of 

 the polyp, the polyps of Isophyllia and Orbicella being among the largest studied. 



The action of the circular sphincter muscle is to bring about the overfolding of the distal 

 region of the column wall upon retraction of the polyps. This occurs in nearly all corals, and, 

 as already observed, it results that the column wall almost completely hides the disk and tentacles, 

 leaving a small central opening over the oral aperture. Circular constrictions may occur in the 

 column wall without any retraction of the disk, in this case the action of the columnar 

 musculature is probably the same as before, but the retractor muscles of the mesenteries have 

 not come into play and drawn downward the oral region of the polyp. 



(t. H. Fowler (lsssa, p. 12) was the first to record the presence of an undoubted sphincter 



