288 THE FRESH-WATER LOCHS OF SCOTLAND 
limited to the extreme north and west, especially the islands ; D. 
gracilis is almost confined to the mainland and nearer islands, and 
appears to be absent from most of the outer islands, both in the west 
and the north. 
Comparatively few of the Desmids are generally distributed ; the 
great majority are limited to the western half of the country, and 
are by far most abundant in Sutherland and the Outer Hebrides. 
Vertical Migration of the Plankton.— Although few observa- 
tions on this subject were made, they demonstrate a diurnal migration 
of considerable amount. Collections were made at short intervals 
during a day and night. It is not easv to demonstrate migration in 
the case of animals which are distributed from the surface to a depth 
of, say, 100 feet or more ; but in the case of Leptodora^ which during 
the day is usually only to be found at some distance below the surface, 
the migration is very evident. On the occasion in question Leptodora 
was only in the deeper collections during the day, and immediately 
after dark it was abundant at the surface. The migration must have 
been very rapid. There was also a \Qvy marked increase in the total 
quantity of the larger plankton organisms during the night — Daphn 'ia 
and Diaptomus gracilis^ at least, having come from the deeper layers 
to the surface. On a subsequent occasion, in calm weather, with full 
moon, no Leptodora was found in the surface collections after 
nightfall. 
Seasonal Change of Form. — It is generally believed that some 
pelagic animals become more elongate in summer, the spines of others 
become longer. Dr Wesenberg-Lund some years ago formulated a 
theory that many plankton organisms changed their form in corre- 
spondence with the changes of temperature throughout the seasons. 
It was supposed that animals which in winter were able to maintain 
their position in the water with comparative ease would be liable to 
sink when the water reached a higher temperature, in consequence of 
the lovi^er specific gravity of the water, unless modified so as to offer 
greater resistance to the downward movement through the water. 
But as the difference of specific gravity produced by the change in 
temperature would be very small, it was afterwards suggested by 
Ostwald that the changes of form were necessitated in a much greater 
degree by the varying viscosity of the water at different temperatures, 
and this view has been accepted by Dr Wesenberg-Lund. 
It has not been possible to throw any light on this subject from 
the study of the Scottish lochs. There were in different lochs all 
the different forms of Asplanclina^ Daphnia^ Bosmina, etc., which are 
supposed to be the seasonal forms, and especially there was an extreme 
range of forms of Ceratmm hirundinella^ but in the only lochs examined 
throughout the year such change appeared to be exceedingly small. 
