SOLITARY CORALS. 221 



forty-eight in number, consisting of six primary, six secondary, and twelve tertiary 

 pairs. The muscle banners are well developed, and the directive pairs, as is usual, 

 have the muscle banners on their outer faces. The general arrangement of the 

 mesenteries and their relations to the septa, mouth, and peristome are shown in the 

 diagram, fig. III., A, B, C, D, E, F, which represent six sections of the same sextant oi 

 a decalcified specimen of Heteroeyathus taken at different levels. The six pairs of 

 primary mesenteries extend further inwards than the rest, and they alone reach the 

 mouth. The six secondary pairs are intermediate in length, and the twelve tertiary 

 pairs are the shortest of all. As stated above, the free edges of the primary 

 mesenteries at and below the lip of the mouth are broadened out to form a T-shaped or 

 Y-shaped "filament" (fig. III., D,JIL). The inner face (i.e., the face directed towards 

 the central cavity of the polyp) is covered with a very thick ciliated epithelium which 

 passes somewhat abruptly into the thinner but still thick epithelium of the lips. 

 Below the level of the mouth the arms of the T- or Y-shaped filament become free 

 from adjacent tissues and may be traced as broad ciliated bands, which in section 

 appear T-shaped, or Y-shaped, or W-shaped, for a considerable distance below the 

 level of the mouth into the gastro-vascular cavity. As long as they retain this size 

 and shape, they are composed almost exclusively of very long attenuated ciliated 

 epithelial cells, whose nuclei stain deeply and are closely crowded together ; there are 

 few glandular elements interspersed among the attenuated cells, and no nematocysts. 

 Towards the bottom of the gastrovascular cavity these bands become smaller, and 

 gradually assume the normal shape of a mesenterial filament ; gland cells become 

 more abundant and large nematocysts make their appearance. At the bottom of the 

 cavity the filament is thrown into a complex coil, and is loaded both with gland cells 

 and nematocysts of the large type. It is impossible to say, from a study of sections 

 alone, whether the coiled masses at the bottom of the intermesenterial chambers are 

 acontia, i.e., free offsets of the edges of the mesentery, or simply coiled filaments. 

 Gardiner (21) describes acontia in Cceuopsammia, but I am inclined to the opinion 

 that in Heteroeyathus the structures that might be mistaken for acontia are only 

 coiled mesenterial filaments. 



The filaments of the secondary and tertiary mesenteries do not reach the axial 

 gastrovascular cavity, and are easily distinguished from the primary filaments. They 

 are much smaller, especially in the upper part of their course, and have the usual 

 kidney-shaped outline in section. Like the primary filaments they appear to consist 

 almost exclusively of ciliated cells in the upper part of their course, but they soon 

 show glandular cells and large nematocysts (Plate III., fig. 15). At the bottom of 

 the gastrovascular cavity the filaments of the secondary and tertiary mesenteries are 

 thickened and coiled like those of the primaries. 



The absence of a definite stomodaeum and the extent and importance of the ciliated 

 bands forming the upper ends of the primary mesenterial filaments are features which, 

 though peculiar, are readily explained by a consideration of their relations to the 



