EXPLANATION OF PLATES. 129 



PLATE 8. 



FIG. 45. Section through part of a mesentery in union with the skeletotrophic tissues where they 

 surround a synapticulum. The mesenterial mesoglcea for some distance from its termination is finely 

 striated, forming a desmoidal process. The skeletotrophic ectoderm on the left side shows faintly a 

 narrow portion of the homogeneous skeletal matrix. 



FIG. 46. Transverse section through an isolated portion of the lining wall of an interseptal loculus. 

 Each lateral extremity was terminated by a synapticulum. The section was stained in iron hajmatoxylin, 

 and shows very distinctly the mucous spaces in the inner endoderm and calicoblast layer. The latter also 

 contains coarsely granular cells and nematocysts. 



FIG. 47. Transverse section through the skeletotrophic layers in their uppermost part, showing the 

 very narrow endoderm and calicoblast layer. The layers are syncytial in character. Irregular canal-like 

 spaces are filled with a deeply-staining substance. Small irregular particles are adherent to the free 

 surface of the ectoderm. 



FIG. 48. Transverse section through a part of the skeletotrophic layers from the lower region. The 

 section gives practically the same details as fig. 46, but is more highly magnified. 



FIG. 49. Longitudinal section through an expanded tentacle. The stem is simple in character, while 

 the swollen knob has a nerve and a muscle layer at the base and nematocysts and gland cells (gr. g. c.) 

 towards the periphery. 



FIG. 50. Transverse section through a portion of a mesentery showing the system of irregular 

 spaces in the endoderm. The Zooxanthellae and cut ends of the retractor muscle fibers arranged along 

 mesogloeal plaitings are also represented. 



FIG. 51. Transverse section through the stomodaeal region of a larva preserved shortly after extru. 

 sion. The six primary pairs of mesenteries are present, but the dorsal directives (iv, iv) and pairs v and 

 vi are yet free from the stomodaaum. The gastro-coelomic cavity is beginning to be established in the 

 nearly solid endoderm; Zooxanthellae are present in the thick stomodasal ectoderm. In this and the 

 next figure the histological details of the endoderm are represented, while the ectoderm is convention- 

 ally depicted. 



FIG. 52. Transverse section through the same larva a little below the stomodaeal region. Mesen- 

 terial filaments are forming at the free end of the first and second pairs of mesenteries. Endodermal 

 thickenings (Prosepten) are present between the larger pairs of mesenteries. 



PLATE 9. 



FIG. 53. Radial vertical section through a young polyp, showing its relationship to the early coral- 

 lum. In the latter are shown the basal plate (. //.), epitheca (/.). tw <> septa, and acolumellar upgrowth 

 (col.) 



FIGS. 54-60. Serial transverse sections through a portion of a polyp, showing the relationships of 

 the mesenteries and septa in the development of a third-cycle entoseptum (b-c) and two fourth-cycle 

 exosepta (a-l>, c-d). The full description of the series is given on p. 100. 



PLATE 10. 



FIG. 61. A free corallum. Natural size. 



FIG. 62. A portion of the same corallum magnified about four times, showing various characteristics 

 of the corallites and their relationships to one another. 



FIG. 63. Upper part of three septa as seen in a vertical surface view. The left and middle septa extend 

 across a calice, but are separated in the middle by the columella and the broken central edges of other 

 septa in union with it. The right septum belongs to an adjacent calice, and is separated from the middle 

 septum by a vertical ridge which represents the line of fusion of an adjacent septum not in the same radius. 

 The two or three vertical rows of large elevations on each septum represent the broken surfaces of synap- 

 ticula ; the smaller elevations are granules. The inner central margin of the left septum shows the denti- 

 culations as they unite with the columellar tangle, and near them the fractured surfaces indicating where 

 an adjacent septum was interruptedly united by its inner margin. 



FIG. 64. Ground surface of a portion of a corallum showing the relationship of a single calice to the 

 six adjacent calices. Surface view. The long axis of the columella represents the principal axis of the 

 calice, the directive septa being at opposite extremities, and not quite in the same plane. 



FIG. 65. Portion of a thin transverse section of a single corallite showing the microscopic structure. 

 The columella, septa, and synapticula seem as if constituted of spheroids fused together; the spheroids 

 represent transverse sections through so many individual trabeculse. 



