REPORT ON CORALS— HYDROCORALLIN^E. 75 



In the next stage observed (PI. IX. SP', OV), the spadix is still further complicated 

 at its margin by subdivision of its lobes, which form a sort of network over one- 

 half of the surface of the ovum, terminating in a fringe of numerous tentacular-like 

 lobes. The ovum is a large ovoid mass, composed of fine rounded particles densely 

 packed together. 



In the next stage observed, the developing ovum has already assumed the elongate 

 cylindrical form of a planula (PL IX. Pi). The stages by which the planula breaks its 

 connection with the spadix were not traced. The earliest planulse observed appeared 

 to be composed entirely of a uniform mass of fine rounded particles, like those 

 constituting the substance of the latest stage seen in relation with the spadix. The 

 formation of the ectoderm appears to take place by delamination. As the planula 

 develops it becomes much elongated, and an outer layer becomes gradually more and 

 more plainly observable on its surface as distinct from a general mass beneath it 

 (PI. IX. P2). The early-formed ectoderm layer (PL XL fig. 5) is composed of closely- 

 set, very fine rounded particles ; whilst the inner mass, or endoderm, is made up of 

 larger transparent oil-globules. As development proceeds, the ectoderm layer thickens 

 and becomes highly transparent, and being colourless contrasts with the more opaque 

 red-pigmented endoderm »within. 



The mature planula measures nearly a quarter of an inch in actual length, and is 

 so long that it has to be doubled up in order to allow of its accommodation within the 

 gonophore sac. The planula has a thick, highly transparent, gelatinous-looking 

 ectoderm, and a darkly pigmented endoderm. It is long and worm-like in form (PL IX. 

 P3). The surface of the ectoderm is marked out into polygonal areas, which are defined 

 on the surface of the planula at an early period of development (PL XL fig. 6). 



A vertical section of the ectoderm of the mature panula (PL XL fig. 7) shows that 

 this thick layer is composed of a transparent gelatinous-looking mass, which is traversed 

 by tracts of small rounded non-transparent elements, which stretch vertically to the 

 surface of the planula, from the surface of the endoderm to that of the ectoderm. These 

 tracts are continued outwards from a layer of similar elements, which rests at the base 

 of the ectoderm, directly upon the surface of the endoderm. The opaque tracts are 

 disposed at roughly regular intervals, and form vertical layers which, rising to the 

 surface of the ectoderm and meeting one another, enclose the polygonal areas already 

 described. In these tracts, apparently out of the opaque elements composing them, 

 numerous thread cells of the larger kind are developed, and are more abundant and 

 thickly set towards the surface of the planula; hence, when the lines enclosing the 

 polygonal areas are viewed from the surface of the planula, they appear mainly composed 

 of rows of nematocysts viewed end on, but partly also of the already described opaque 

 rounded bodies (PL XL fig. 8). Some of the nematocysts contained in the ectoderm 

 of the mature planula were observed to have their contained threads fully developed. 



