OF THE ENC4LISH LAKE DISTRICT. 427 
Diurnal migration o£ the species was strongly marked at the time of my 
visit. On three successive nights surface plankton samples were taken over 
the shallow and medium (50—70 feet) depths, and in each case Limnocalanus 
was found in much greater numbers than during the day. The figures in 
the table do not bring out this fact so clearly as would have been the case if 
absolute numbers could have been given, but the difference between day and 
night plankton was quite obvious and striking on the spot. 
So far as concerns seasonal distribution, the facts with regard to Limno- 
calanus in Ennerdale are somewhat uncertain. Ekman has shown that in 
the Baltic lakes tliere is only one breeding-period — in autumn. Young 
appear first in March, and by May the first-hatched young are fully grown. 
But there is then a period of about five months during which little or no 
growth takes place and the genital organs remained undeveloped. Sexual 
maturity is reached in autumn, and breeding begins in November at a 
temperature of about 7° C. The adults of the previous year die off in 
spring and all have disappeared by May. 
Dr. Pearsall's collections may be regarded as indicating a similar life-cycle 
in Ennerdale, but are by no means conclusive. Neither young nor adult 
were taken on Feb. 18, 1922, but two adults and many young were found in 
a sample from the shallow end of the lake on April 20, though, curiously 
enough, none at all were taken in a deep-water collection on the same day. 
In June and July no young, but a very few adults, were taken ; but in 
October adults were common in 60 feet of water. It seems safe to say that 
young are only found in spring, and that adults are abundant, and even the 
commonest plankton species, in September and October. 
The interest of this species lies more, however, in its geographical distri- 
bution and its origin as a member of the fresh- water fauna, and it is necessary 
to go into this question in some detail since- its occurrence in Ennerdale 
raises a particular difficult geological problem. 
Ekman has dealt exhaustively with the structure and distribution of 
L. macrurus (1913), particularly with reference to the Baltic area, and his 
results may be summarized as follows : — 
1. L. macrurus of fresh-water lakes is not specifically separable from 
L. grimaldii, which inhabits brackish water in the Baltic and Caspian Seas 
and along the coasts of Alaska and Siberia. 
The differences are : — 
(a) Shape of head, the dorsal contour being more swollen in L. macrurus. 
(6) In L. grimalcUi the last thoracic segment is rather triangular or 
pointed, or may have a minute hook-like projection, whereas it 
is rounded in L. macrurus. 
(f) The antennae are longer in L. grimaldii. 
(rf) The furcal rami of L. grimaldii are longer than in L. macrurus. 
While the two extremes are readily distinguished, they are united by 
local varieties presenting every intermediate condition. 
31* 
