FRESH-WATER PLANKTON 
351 
Fahr. ; and Loch nan Eun (North Uist), 11th May 1904, surface 
temperature 52° '0 Fahr. — the Lewis and North Uist lochs, at a 
temperature practically identical with that of the Shetland lochs, had 
the large and slender form of Ceratium. And as regards the develop- 
ment of a fourth spine, it is largest when the temperature is high, and 
generally speaking its development progresses simultaneously with the 
elongation and divergence of the second and third spines ; and yet in 
a number of cases — e.g. Loch Raonasgail (Lewis), Loch an lasgaich 
(North Uist) — in lochs at a high summer temperature the fourth spine 
was very ill developed, though the other spines were long and the 
organism large and slender ; and again, in the small stumpy forms of 
some Shetland and Orkney lochs the fourth spine was relatively well 
developed, and was actually larger than that in some of the slender- 
formed Ceratiums of Lewis and North Uist (compare Lochs Kirbister 
and Harray with Lochs nan Deaspoirt and nan Eun). 
It may be that the Ceratiums of each loch in any area whatsoever 
will vary according to the temperature of the water, becoming large 
and slender and four-spined if the summer temperature be high 
enough, and small, stumpy, and three-spined in winter, and that the 
differences of form in lochs of like thermal conditions are due to a 
varying sensitiveness of the species, or to the presence of some unknown 
restraining factor in varying degree. 
EXPLANATION OF THE FIGURES 
Plate X 
Fig. \. Juvenile Daphnia from Loch Raoinavat (Lewis). 
Fig. 2. Adult 
Fig. 3. „ „ „ Valtos 
Fig. 4. Juvenile „ „ ,, 
Fig. 5. Late embryo of a Loch Valtos Daphnia. 
Fig. 6. Young Daphnia from Loch na Craobhaig (Lewis). 
Fig. 7. Older 
Fig. 8. Egg-bearing Daphnia from Loch a' Chlachain (Lewis), 
Fig. 9. n J) nan Deaspoirt (Lewis). 
Fig. 10. Microcephalic adult from Loch Fadagoa (Lewis). 
Plate XI 
Fig. 1. Juvenile Daphnia from Loch an Tomain (North Uist). 
Fig. 2. Egg-bearing Daphnia from Loch an Tomain (North Uist). 
Fig. 3. Juvenile Daphnia from Loch an lasgaich (North Uist). 
Fig. 4. Egg-bearing form from Loch Olavat (Benbecula). 
Fig. 5. Adult Daphnia of Loch Vieragvat (North Uist). 
Fig. 6, Juvenile Daphnia of Loch Vieragvat ,, 
Fig. 7. „ Skealtar „ 
