466 
MR. H. N. MOSELEY OR THE STRUCTURE OF THE STYLASTERID2E. 
but dotted all through their mass with sparsely-scattered oil globules of various sizes 
(Plate 42, OY). The reflection of the ectoderm at this stage still covers the ovum 
within its cup. A space is enclosed all round the gonophore by this ectodermal mem- 
brane, between the margin of the spadix and the ovum. This is filled by a perfectly 
transparent fluid. 
In the next stage observed (Plate 42, SP', OY') the spadix is still further com- 
plicated 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 (Plate 42, Pi). The stages by which the planula breaks 
its connexion with the spadix were not traced. The earliest planulee 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 
developes 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 
(Plate 42, P 2). The early-formed ectoderm layer (Plate 44, 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 
(Plate 42, 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 
(Plate 44, fig. 6). 
A vertical section of the ectoderm of the mature planula (Plate 44, 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 
