1 79 
Observations on the Pyrocystece. 
regions, but they are usually confined to the open sea; both are of 
large size, P. pseudonoctiluca reaching a size of 0.5 mm., while 
P. fusiformis is twice as long. Their luminous power is very 
marked, the diffused luminosity of the open sea in warmer regions 
being mainly due to species of Pyrocystis (Peridineai playing only 
a subsidiary part), as that of coastal waters is largely due to 
Noctiluca. 
In spite, however, of their large size, great abundance and striking 
luminous power, and also of the interest which attaches to the 
question of their systematic position, the members of this group have 
received but little attention, chiefly owing, no doubt, to the difficulty 
of obtaining suitable material. The observations here detailed were 
made some years ago on material obtained by pumping sea-water 
through bags of fine miller’s silk, chiefly during a voyage to the 
West Indies. The results have been kept back with the hope of 
obtaining more material for the purpose of elucidating further 
such points as the method of reproduction and of nuclear division, 
but as this has proved fruitless, and the long promised memoir by 
Brandt, on the Pyrocysteae collected by the Hensen expedition in 
1892-3 (which would, no doubt, render their publication unneces¬ 
sary), seems unlikely to appear, they are put forward in the 
present form. 
Pyrocystis Pseudonoctiluca, Wyv. Thoms. 
Structure .—A good general view of this form was given by John 
Murray (6) in 1876 . This figure is later reproduced, with others, 
in the “Challenger ’ narrative, and there the general arrangement 
and colour of the protoplasm, and the cellulose wall are noted 
and several stages of division figured, the presence also of a large 
nucleus is mentioned, but no detailed account is given of the structure 
of the organism. The cell is perfectly spherical in form, varying 
considerably in size, but reaching a diameter of 0.5 mm. (sometimes 
slightly more) and being just visible to the naked eye. Surrounding 
the whole is a very distinct and beautifully translucent glassy mem¬ 
brane of extreme tenuity not measuring more than 0 . 5 //, in thickness. 
The w r all gives clearly the reactions of cellulose, it dissolves completely 
and very readily in ammoniacal copper oxide, it disappears without 
leaving any trace in strong sulphuric acid, and with chlor-zinc- 
iodine gives not a blue but a reddish purple colour; the colour pro¬ 
duced in the last reaction suggests that the cellulose may be present 
in a slightly modified form. When the organism was first dis¬ 
covered, the wall, owing, no doubt, to its glassy appearance, was 
