440 MK. K. GUENEY ON THE CRUSTACEAN PLANKTON 
large Swiss lakes, however, it appears to be a regular member of the 
plankton, and Burckhardt has described the form thei'e found as a distinct 
species — S. limnetica, characterized chiefly by the absence of a nuchal organ. 
In the Lake District it was found in small numbers in the majority of the 
collections from Derwentwater, so that it may probably be regartled as really 
limnetic there, whereas the few records of it from Bassentbwaite may be due 
simply to migration from the littoral region. The same is the case with 
Esthwaite and G-rasmere. On the other hand, my experience of night 
collecting in Ennerdale seems to show that this species; though remaining 
in the littoral region during the day, may take to the open water at night. 
(See p. 422.) 
All the specimens from the Lake District were of the typical form, with 
well-developed nuchal organ. 
HoLOPEDiUM GIBBEBUM, Zadclach. 
The distribution of Holopedium in the Lake District is somewhat peculiar. 
It is recorded by Beck from Easedale Tarn (915 feet) and from Windermere, 
and it also occurs in Grasmere (P. A. Buxton, Sept. 1912) and Eydal Water 
(D. J. Scourfield). In Uilswater it was found in Sept. 1912 and on Aug. 20, 
1920, but not in any other collection made by Dr. Pearsall. In Ennerdale 
it was present in very small numbers in three out of seven collections taken 
by him, but was abundant in my own collections in Sept. 1922. So far as 
is known at present it does not occur in any other lake in the district. 
Easedale Tarn, Grasmere, Eydal, and Windermere form part of the same 
drainage system. The fact that it has so rarely been taken in Windermere 
mav possibly be due to its being occasionally carried into the lake, but not 
becoming established there ; but it is more probable that it is not uniformly 
distributed in the lake, and also that, being a deep-waier form, it is not 
taken in surface plankton. In Ennerdale it is fairly evenly distributed in 
both shallow and deep parts, but it is rare on the surface during the day- 
time, preferring deep v\ ater. At night, however, it is abundant on the surface. 
This preference for the deeper layers of water does not appear to be 
generally characteristic of the species, since Scheffelt (1908) states that it is 
found at the surface at all times; but he also found it to be unevenly dis- 
tributed and to occur in swarms. Lilljeborg notes the same thing in 
Sweden. 
It is generally regarded as a " Glacial relict," being confined to northern 
regions or to cold water. 
The ijenus Daphnia. 
The increase of knowledge of local and seasonal variation among the 
limnetic Daphnias has led to the general recognition that any division into 
well-defined species is impossible. All are therefore regarded as belonging 
to one species, D. longispina, within which an almost infinite number of 
