AMPHIDINIUM OPERCULATUM AT PORT ERIN. 391 
Irish Sea during the last four years, and which have been examined in minute 
detail by Mr. Andrew Scott, A.L.5. 
Thinking it might be present in the shallow water close to the edge of the 
beach, Mr. W. Riddell and I took some hauls of the tow-net from a punt 
worked backwards and forwards in a few inches of water as near as we could 
eet to the discoloured sand, but the gathering, although it contained fine 
sand and mud, showed no trace of our Dinoflagellate. It may be noted here 
hat although the size of the Amphidinium, 0-05 mm. in greatest diameter, is 
such that it can slip through the mesh (averaging about 0*08 mm.) of the 
finest plankton silk (no. 20), still so much clogging of the meshes always 
takes place in such hauls, and so many other smaller organisms and particles 
of mud are retained, that it is certain that had the Amphidinium been 
present in any quantity in the water it would have shown up in the 
gatherings. 
Careful scraping of the sand showed that the Dinoflagellates were only in 
and on the surface-layer, and therefore could not be regarded as coming up 
from below. It occurred to us that possibly they might be fresh- or brackish- 
water forms derived from the land; but we ascertained that the little 
siream in the centre of the bay, which in wet weather overflows on to the 
beach (at other times it is conveyed into the town sewer), had not, on account 
of the unusually dry season, sent any water to the beach for some weeks. 
Moreover, on experimenting with the living Amphidinium in the laboratory, 
we found that while it lived well in sea-water it died at once in fresh and 
survived for а few days only in brackish water. "amples of the Amphi- 
dinium kept in shallow dishes of wet sand at the Biological Station in a few 
days showed such profuse growth that the sand was covered by a dark- 
coloured layer, the water became impure, and eventually all the Dinoflagellates 
died off. | 
Observation under the microscope shows that although this is a singularly 
active Dinoflagellate, circling round and round with great vigour, so that a 
drop of sand and water containing a number of the organisms presents а most 
animated picture under а low power magnification, still the Amphidinium 
seems to be actually attracted to the sand-grains and associated with them. 
The sand-grains in the field of view are always peppered over with a number 
of specimens of the Amphidinium (Pl. 18. figs. 2, 3, 4), and if individuals be 
watched they are seen after swimming round to come back to rest on a 
sand-grain and remain there for a time before starting off on another 
excursion. If they are thus constantly associated with sand-grains or other 
solid particles, and never swim more than a microscopic distance from such 
a resting-place, that may account for the fact that we have never found them 
in our plankton gatherings. 
Amphidinium operculatum is also, however, positively heliotropie, con- 
regating in quantity on the lighter side of the dish in the laboratory, and 
or 
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2 е2 
