426 ME. R. GURNKY ON THE CRUSTACEAN PLANKTON 
by Diapliaaosoma, while the rocky tariTS oE Coniston have a very scanty 
fauna, both of species and individuals. The brilliant colour, either red or 
blue, of D. gracilis in these high tarns is very striking. 
The means of dispersal of the species is a problem of some obscurity. All 
the tarns are of comparatively recent origin, owing their existence to 
morainic dams or rock-falls, and each, as a rule, occupies the head of a 
lateral valley. 
Keilhack, studying high-lying waters in the Dauphine Alps, concluded 
that distribution of species w:is effected by migratory birds, and that wafer- 
sheds rarely, if ever, exert any influence on distribution. While it is 
impossible to offer any other satisfactory explanation, the agency of birds, 
especially in the Lakeland area, seems to be of doubtful efficacy. A detailed 
survey of the tarns of this compact and uniform district, with special 
reference to the dissemination of species, would probablj^ lead to results of 
great general interest. 
Notes on some of the Species. 
LiMNOCALANUS MACRUKUS, Sars. 
The occurrence of this fine Centropagid in Ennerdale is rather a startling 
discovery, and raises problems of exceptional interest. It was first recognized 
in a collection made by Dr. Pearsall on Sept. 23, 1921, the sample con- 
taining five specimens. As a result of this discovery Dr. Pearsall arranged 
for a series of collections to be made in the lake, and I also visited it myself 
on Sept. 27, 1922, but, although I am able to give some account of its 
distribution in the lake and its seasonal cycle, the facts are by no means 
complete, and can only be ascertained by quantitative methods with suitable 
apparatus. So far as information goes at present, the species is verj' abun- 
dant in the lake and occurs in every pait of it, at all events in autumn, which 
is the period of maximum development ; but it is far more abundant in the 
deep eastern trough, and occurs only in small numbers in the shallow western 
end. All previous observations, bj^ Ekman and others, agree in showing 
that L. macrurus is only exceptionally found at the surface, and is almost 
entirely confined to water below a depth of 5 metres. Further, the maximum 
temperature of water in which it lives is 14° C, but even when the tempera- 
ture is lower than this it does not come to the surface. In Ennerdale this is 
not the case. The first collection in which it occurred was made on the 
surface (in stormy weather it is true), the temperature of the water being 
14-4° 0., and rny own collections between Sept. 27 and 29 proved that it lived 
at that time in considerable numbers at the surface, although it certainly was 
more abundant in deeper water, down to about 80 feet. It is impossible to 
give exact figures, since I had no closing net at my disposal, but the figures 
given in Table 2 (p. 417) will serve to show the distribution at that time 
with, I believe, a sufficient approximation to the ti'uth. I was imfortunately 
unable to take temperature observations. 
