PRESIDENTIAL ADDRESS, 509 
have they not all by this time reached the same grade of perfection? Why not 
indeed, unless because every new group is less hampered by tradition, much 
of which must be discarded with the new departure; and some of its energy is 
set free to follow up this new course, straight, with ever-growing results, until 
in its turn this becomes an old rut out of which a new jolt leads once more into 
’ fresh fields. 
The following Papers and Reports were then read :— 
1. The Hebridean Diazona. By Professor W. A. Herpman, F.R.S. 
Professor Herdman exhibited with remarks some specimens of the rare 
compound Ascidian Diazona violacea, which he had dredged recently in the 
Hebrides from his yacht Runa. When alive the colony is bright green, but when 
preserved in alcohol it becomes violet in colour. Specimens preserved in a 
solution of formol remain, however, unchanged in the green condition; and the 
two colours were shown to the Section side by side—the green (known formerly 
as ‘Syntethys hebridica’ of Forbes and Goodsir) and the violet (Diazona 
violacea Savigny), the form usually supplied to museums by the Zoological 
Station at Naples. The Hebridean and the Mediterranean forms are un- 
doubtedly the same animal, and are exactly alike in minute structure. Pro- 
fessor Herdman was able to show that Hebridean specimens dredged from deep 
water when kept alive in sunlight changed their colour and gradually became 
darker and bluer, and finally acquired a slightly violet tint, although still in 
the living condition. The green colour is not due to chlorophyll, but to an allied 
pigment which has been described as ‘ Syntethein.’ The chemistry of the change 
of colour to violet has not yet been explained. 
2. On the Bionomics of Amphidinium operculatum Clap. and Lach. 
By R. Douauas Laurig, M.A. 
A. operculatum occurs as brownish green patches on the sand just below high- 
water mark. These patches are sometimes clearly visible a hundred yards away, 
and yet a few hours later they may have so completely vanished, to about an 
eighth of an inch beneath the surface of the sand, that one would not suspect 
its presence on the shore at all. It was first recorded for the British Islands 
by Professor Herdman, who found it at Port Erin, Isle of Man, in 1911. 
I found it on the Cheshire coast at Hoylake on February 9, 1913, and have 
made daily observations of its movements since and find that they show three 
periodicities. 
(a) A daily periodicity. Very striking during the spring. For instance, 
during the latter half of February the patches were very evident on the 
surface of the sand on most days until 10 a.m., then they went gradually just 
below the surface, becoming entirely hidden from view at about 11 a.m., and 
so remaining until shortly after noon, when they once more gradually appeared, 
reaching a maximum about 2.0 p.m. which they retained until about 4.0 p.m. 
and then again disappeared. Two possible explanations of this daily periodicity 
suggest themselves: (i) expression of habit; (ii) a new direct response of the 
organism each day to daily recurring environmental stimuli. Experiments have 
been made which lead to the conclusion that the latter is the correct view, 
altered experimental conditions leading at once to correspondingly altered move- 
ments. The stimuli more immediately concerned are those of light and tide. 
Maximum light is by no means the optimum for Amphidinium, the optimum is 
a dull light. The reaction of the organisms is strongly negative to strong light, 
that is light which takes less than, say, ten seconds to colour the silver paper 
exposed in a Watkins ‘Bee’ photographic exposure meter; on the other hand, 
their reaction appears not negative but merely neutral to darkness. The con- 
clusions based on experiment were confirmed by finding later in the year, as the 
days become longer, that the periodicity gradually changed, the morning 
maximum becoming earlier and the afternoon maximum later, so that in the 
latter half of May the brown patches were found to appear about 4.30 p.m. and 
in the latter half of June at about 7.0 p.m. The other important factor with 
which its daily movements are correlated is the tide, and the addition of this 
