FRESH-WATER PLANKTON 
347 
the reduction of the mucro and rostrum at the moult is plainly seen. 
This Bosmina is a small form of B. ohtiisirostris^ s. str. 
Cerat'mm hirimdinella, Midler. — The Peridineae are represented 
by various species in our lochs, the most abundant being Cerat'mm 
hirundinella. This organism varied very considerably even within the 
same area, and although on the whole the Shetland Ceratium was 
widely different from that of Lewis, these are not to be considered 
as distinct and fixed varieties, but probably as temporary adaptation 
forms ; it would appear that the organism is exceedingly plastic, and 
its numerous forms are, I think, the expression of varying physical 
conditions, such as temperature. 
In the small island of Lismore all three lochs (Fiart, Kilcheran, 
and Baile a' Ghobhainn) had this species in great abundance, but, 
curiously enough, the Ceratium of Loch Baile a' Ghobhainn was quite 
unlike that of Lochs Kilcheran and Fiart. The tow-nettings were 
taken at the same time (13th to 16th August 1904) ; the lochs are all 
moderately deep — over 20 feet in mean depth, — and the surface 
temperature was fairly high, being in Loch Baile a' Ghobhainn 62° '7 
Fahr. ; unfortunately, we have no record of the temperature of the 
other two lochs, but it is very unlikely that it could be appreciably 
lower than that of Loch Baile a' Ghobhainn, especially as the latter 
is larger and deeper. All the specimens of Ceratium Mrimdinella 
found in Loch Fiart and Loch Kilcheran were small, stumpy, and 
rather spinose ; the fourth spine was either absent altogether, or only 
very slightly developed ; the first spine was short, as also were the 
second and third, which, moreover, were either parallel or not much 
divergent (Plate XV. figs. 1 and 2). In Loch Baile a' Ghobhainn all 
the specimens were large ; the spines were long and smooth, the 
fourth being usually quite as well developed as the third ; the 
third and fourth spines were widely divergent from the second 
spine (Plate XV. fig. 3). I cannot give any explanation of these 
marked differences in lochs where the physical conditions were appar- 
ently identical. 
In Loch Frisa (Mull), examined 17th August 1904, the surface 
temperature being 59 '1 Fahr., the Ceratium was much like that of 
Loch Fiart ; there was some variation in the length of the fourth 
spine, but it was always short or nearly absent (Plate XV. figs. 4 
and 5). 
Ceratium hirundinella was rare in the lochs of North Uist and 
Benbecula, though it occurred in about half the lakes examined. 
On 9th May 1904, in Loch Skealtar, with a surface temperature 
of 47° '7 Fahr., the majority of the individuals were rather small ; the 
spines were short, the fourth spine being ill developed or wanting ; a 
few specimens were more slender, the fourth spine was present, and 
