416 • ME. E. GUENEY ON THE CRT'STACEAN PLANKTON 
Dr. Pearsall suggests that the inipoverislinient of the plankton of Biissen- 
thwaite may be due to the silt from lead mines washed into the lake during 
the past fifteen or twenty years. It is quite possible that to this cause is due 
the difference between the present plankton and the list given by Miss Pratt 
in 1898. 
Group III. is also by no means a natural group from the point of view of 
the Crustacea, although the fact that it is only in the three lakes — Oonistou, 
Windermere, and (Jllswater — that Daphnia of the galeata-i.ovm. occurs, may 
indicate a similarity of conditions. The presence of Holopedium, either 
regularly or occasionally, in Ullswater and Windermere may also be regarded 
as evidence of similarity, though the status of the species in the district is 
rather doubtful. It seems to me that, if any grouping is to be attempted on 
the basis of the crustacean plankton, Ullswater and Hawes Water should be 
included in the series of primitive lakes, while Windermere, with its Dia- 
pJianoso7na, Ceriodaplinia, and Cyclops leucharti, belongs more nearly to the 
Esthwaite-Grasmere series. Coniston, with its dominant crustacean plankton, 
comes near the primitive type, while Derwentwater and Bassenthwaite lead 
on towards the more evolved type of Windermere and Esthwaite. 
It is impossible, without greatly extended knowledge of the food require- 
ments of the Entomostraca, to determine what are the factors controlling 
their distributions in an area such as this. The evidence is conflicting. 
Birge (1898, p. 353) states that iJaphnia feed readily on filamentous Diatoms 
(e. g. Melonira) and also on Aphanizomenon and Anabcena, while Diap>to7nus 
prefers Anabcena and Aphanizomenon to Diatoms. On the other hand, more 
recent investigations lead to the supposition that but little correspondence 
will be found between the distribution of the Entomostraca and the net 
phytoplankton, since the Entomostraca appear mainly to leed upon the 
minute nannoplankton, which is not taken in the plankton net. Woltereck 
^^1908) stated that the net plankton is far too large to be taken in by Dajjhiia, 
their food consisting partly of the finest detritus, but mainly of the nanno- 
plankton. The latter he found to be more abundant in the Lunzer Obersee, 
where Daphnia was more flourishing, than in the Untersee, and attributed 
this to the greater organic content of the water. This result was confirmed 
by experiment, since it was found j)Ossible to maintain limnetic Daphnias in 
health on a diet of a pure culture of Chlorella. 
Naumann * concluded that it was not the Algse themselves but the detritus 
produced from them, and the snialler Flagellates, which provided the food 
for the Crustacea, and that the dust-fine organic detritus or peritripton, which 
may be introduced into the lake from without, may be of the utmost 
importance in the lime-free Palaeozoic regions. In such lakes the zoo- 
plankton is greatly in excess of the phytoplankton. The lakes of the 
* I have not been able to consult Naumann's paper, but rely on a review of it in Arcb. 
f. Hydvob. xii. 1920, p. 83.o. 
