ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
709 
in the free larva, and a continuous mesodermic membrane is formed 
which envelopes all the elements which were primitively free in the 
cavity of the larva, as well as all the products of histolysis. The cystid 
is also provided at its aboral pole with a thickened disc consisting of 
two layers ; of these, the outer and ectodermic arises from a thickening 
of the aboral ectoderm, while tbe internal layer, which is mesodermic, 
arises from a corresponding thickening of the mesoderm. This meso- 
ectodermic disc is destined to form the polypide. 
Preceding investigators have thought it probable that the aboral 
organ of Flustrella takes part in forming the polypide, but it really does 
nothiug of the kind. The polypide arises altogether from the meso- 
ectodermic disc, which is formed independently of the aboral organ. 
The vesicular rudiment results from a simultaneous invagination of 
the two layers of the disc, and this subcuticular invagination is pro- 
duced after the degeneration of the aboral organ. 
The internal (ectodermic) layer of the rudiment forms the lophophore, 
the outer wall of the tentacles, the nerve-ganglion, the internal lining 
of the invaginated sheath, the pharynx, the rectum, and the mesenteron. 
The outer (mesodermic layer) forms the inner wall of the canals of the 
tentacles, the outer covering of the invaginated sheath, the retractor 
muscles of the polypide, the occlusor muscles of the chamber, and the 
investment of the digestive-tube. 
South Australian Polyzoa.* — Mr. P. H. MacGillivray publishes an 
additional list of Polyzoa, founded on a collection of 119 species, 71 of 
which were not in his previous list. Three appear to have been hitherto 
undescribed. 
Arthropoda. 
Histological Arrangement of Pigment in Eyes of Arthropods*!— 
Mademoiselle M. Stephanowska has investigated the histological arrange- 
ment of pigment in the eyes of various Arthropods under the influence 
of direct light and complete darkness. She finds that light and darkness 
do influence the arrangement of the pigment, and that this is shown by 
the movement of the pigment-cells and the pigment-granulations. The 
general characters in complete darkness are — the pigment is not distri- 
buted uniformly, but there are large, very compact masses, chiefly at the 
base of the cones ; the pigment-cells are more contracted, and are con- 
sequently more distinct ; they also cover a smaller number of the optic 
elements, and these latter are more distinct than they are after exposure 
to light. In good light the pigment is much more uniformly scattered, 
and there are but rarely localized masses ; the pigment-cells are elon- 
gated towards the cornese and towards the retinulae. In consequence 
of this, the refractive and sensory elements of ‘the eye are less distinctly 
visible than in darkness, and the contours of the pigment-cells are them- 
selves also less distinct. The pigment seems paler, for it is extended 
over a larger surface. 
In some Insects the pigment, under the influence of strong light, is 
changed into droplets of fatty appearance, the size and arrangement of 
which vary much in one and the same eye. This phenomenon was 
* Trans. Roy. Soc. South Australia, xiii. (1890) pp. 1-7 (1 pi.), 
t Rec. Zool. Suisse, v. (1890) pp. 151-200 (2 pis.). 
