4 G. HEBERT FOWLER. 



of the polyp cavity, but occur as discontinuous ridges (Fig. 6, Ab). 

 In every polyp, however, either an axial or abaxial septum is 

 present, which enables the orientation of the polyp to be effected as 

 in the Alcyonaria. (These terms, axial and abaxial, are used in 

 preference to the ordinary and misleading "dorsal " and " ventral," 

 and were suggested originally by Professor Milnes Marshall, "Trans. 

 Koy. Soc. Edin.," 1883.) 



There is no columella, but often the axial and abaxial septa fuse, 

 low down in the polyp cavity, so as to divide it into two equal 

 halves (Fig. 3, a'), in a manner suggestive of the " median plate " in 

 Pocillopora and Seriatopora figured by Professor Moseley (3). 



The costce bear apparently no relation to the septa in the well- 

 grown colony, whatever may have been the case in the founder-polyp. 

 Not only is no connection traceable between them in a transverse 

 section of the branch, but even in a single polyp standing off from 

 the stem, where the number of septa is under the most favourable 

 conditions but six, about twenty costre surround the calicle. 



B. Anatomy. — The whole of the corallum is covered externally 

 by a definite body ivall of ectoderm, mesoderm, and endoderm 

 (Fig. 6, ext. b. w., Fig. 4, ed. me. en.), immediately beneath which 

 lie, as in Rhodopsammia, external longitudinal canals parallel to the 

 long axis of the corallum (Figs. 3, 4, 6, c 1 ). These, however, are 

 not the result of the same anatomical relations in both cases ; in 

 Rhodopsammia, lamellae of mesoderm with a layer of endoderm on 

 each side are given off from the external body wall, and unite with 

 the endoderm and mesoderm which clothe the exterior surface of the 

 theca ; and into the canals thus formed project the costse. In M. 

 Durvillei, the layer of endoderm and mesoderm which is immediately 

 apposed to the exterior surface of the corallum, rises in a ridge 

 towards the external body wall; and at the points where these 

 layers meet and fuse are formed the costse, i.e., in the angle of the 

 mesoderm ; and therefore between the costse lie the canals. A com- 

 parison of Fig. 4 with (4) Fig. 17 will make clear the anatomical 

 difference. 



There is thus no trace of any structure resembling the " peripheral 

 continuations of the mesenteries of von Koch." 



These canals appear to open over the lip of the calyces into the 

 polyp cavities ; they are connected with each other transversely 



