H7 



As the series of sections show (Fig. III.), the primary and 

 •secondary septa fuse with one another by their edges, so as 

 to close in the axial fossa at a certain depth with a mass of 

 trabeculae (Fig. III. C). Lower down the secondary septa 

 become smaller, and the fossa is closed in by processes from 

 the primary septa alone (Fig. III. B.). Indeed the latter pro- 

 cesses form almost a solid mass closing in the axial fossa. 

 Still lower, the primary septa themselves become reduced so 

 that the axial fossa is quite open (Fig. III. A.). The tissues 

 of the different segments between the primary septa fuse 

 across, and place the whole coelenteron in free communica- 

 tion. In the lower half of the polyp the coelenteron is, how- 

 ever, much reduced — in many places almost obliterated — 

 by the convolutions of the mesenterial filaments. 



Besides the above points, the bearing of which will be 

 considered later, there is little to note about the polyp. The 

 •enormous thickness of the body-wall, as seen in the camera 

 lucida drawings (Fig. III.), is due to the relatively great 

 thickness ever3^where of the endoderm. The layer is coarsely 

 granular, and somewhat similar to the same layer at the base 

 of the mesenterial filaments in the adult (p. 142, and fig. 8) ; 

 cell outlines may, however, be distinguished in some places 

 at the free edge of the layer. Fat cells are present here and 

 there, and foreign bodies (principally algal) are found near 

 the bases of the mesenterial filaments, showing that the polyp 

 has been actively feeding even in its present condition. The 

 typical aborted nematocysts of the adult endoderm (p. 142 and 

 figs. 3 and 9) may be found anywhere, but occur mainly near 

 the septal edges. They only differ in being somewhat smaller 

 than in the adult. On the sides of the mesenteries the endo- 

 derm only differs in being thicker and more granular. The 

 flattened plates of the structureless lamella have but com- 

 menced to form. The muscular fibres are as yet more or less 

 isolated from one another, except at the base of the polyp, 

 and do not form the characteristic square blocks usually seen 

 in a transverse section. 



The mesenterial filaments are of the same structure as in 

 the adult, but rather more granular. The structureless 

 lamella is extremely thin, in many places scarcely existing 

 as a distinct layer. 



The calicoblastic ectoderm varies from a columnar to a 

 cubical facies, and is everywhere a well-defined hyaline 

 layer with little or no granulation. The " desmocytes," 

 except at the base of the polyp, are not yet attached to the 

 structureless membrane. The whole layer is noticeable for 

 its hyaline appearance and well-defined edge against the 

 corallum, never having a ragged surface except near the 

 attachments of the mesenteries. Above the corallum the 



