FRESH-WATER PLANKTON 
341 
much the same throughout. According to Mr T. Scott, the galeated 
forms in Loch Oich and other lochs of the Highland region, whilst 
exhibiting some amount of variation, nevertheless on the whole 
preserve a large crest up to and throughout the egg-bearing period, 
and the males are all provided with a prominent crest ; on the other 
hand, in the lakes of the islands under consideration I have never seen 
crested males, though in the very few cases where males were seen 
the females were galeated. It appears, therefore, that in some of the 
mainland lochs Daplinia galeata is a permanent form, so far as the 
summer season is concerned. 
In the lakes of Lewis I have occasionally noticed that the Daphnias 
are only represented by two stages {e.g. the very galeate form without 
eggs, and the slightly galeate form with eggs), without many inter- 
mediates, as if the brood-formation were simultaneous throughout the 
adult Daphnia population, a phenomenon well known in the lakes of 
the Continent ; but this is unusual in Lewis, and apparently a Daphnia 
produces in succession several series of summer eggs, with the result 
that Daphnias of all stages of the life-history are to be found at any 
time. In several of these lochs the older Daphnias had a purple 
coloration on some part of the valves, due to the deposit of pigment 
in granular form between the two chitinous layers of the valve ; often, 
too, the internal body-tissues were tinged with a suffused purple colour. 
But coloration was most marked in the Diaptomus of Lewis, which 
was almost invariably a vivid blue, but occasionally was red. 
Ephippium-bearing females were only rarely seen ; in fact, I have 
taken them only in Loch Fadagoa (9th August), and the heads were 
rounded. I was unable to discover the males. 
The adult round-head Daphnia of Lewis is about 1*8 mm. in 
length (including spine), the eyes are moderate-sized or large, and it 
agrees fairly well with the variety obtusifroiis, Sars, or in a few cases 
the variety microcephala^ Sars. 
The Daphnias of North Uist and Benbecula were on the whole 
very similar to those of Lewis. The younger adults had a rather 
large and slightly galeate head, whilst the body was small and carried 
only one summer egg ; the older ones had a smaller but rounded head, 
and the body was larger and carried three, four, sometimes six, or 
even eight summer eggs — the one figured from Loch Vieragvat had 
eight but this is rather exceptional. 
There were considerable differences in the surface temperatures of 
the lochs, as North Uist was visited in the early summer, just when 
the thermal changes were most rapid. In Loch Skealtar, examined 
9th May 1904, the surface temperature was 47° '7 Fahr. ; its adult 
Daphnias had rounded heads, many of them having a faint indication 
of a peak, whilst the young were slightly but not markedly galeate. 
