FRESH-WATER PLANKTON 
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Daphnia longispina^ O. F. Mliller. — The Daphnias of all the 
lakes under consideration are varieties of this protean species, and 
it occurs under different forms in the several areas. There is not 
sufficient material to justify a definite answer respecting the amount 
of seasonal variation that obtains for this species in our lakes, but 
the evidence, so far as it goes, would imply that seasonal variation, 
if it occurs at all, is not very pronounced, and I am inclined to regard 
the varieties dealt with below as fairly permanent local forms. 
In the island of Lewis, a tow-netting from any one loch contains 
Daphnias of varied shape and size; usually the majority are slender 
forms with long posterior spines and galeate heads, whilst a few are 
larger and stouter creatures with rounded and protruding foreheads 
and shorter posterior spines. If young are present, they are relatively 
stout forms with slightly galeate heads and long posterior spines. 
When just about to leave the brood-pouch of the parent, the young 
Daphnia has a perfectly rounded head with no indication of galeation. 
Only on one occasion have I met with an exception to this rule : in 
Loch Valtos (15th August 1903) a late embryo, still in the parent's 
brood-pouch, had just the first beginnings of a galea on the head. In 
the same tow-netting there were advanced embryos which had perfectly 
rounded heads, so far as the outer shell was concerned ; but within 
that shell the very delicate skin was slightly peaked, and at the 
succeeding moult the creature would have a correspondingly galeate 
head. But, as a general rule, however galeate the parent may be, 
its newly hatched young have rounded heads. Nevertheless it very 
quickly becomes galeate, and at every moult up to a certain stage 
the Daphnia becomes more pronouncedly so. Eventually the galeated 
Daphnias begin to produce parthenogenetic eggs, the number of 
individuals in a brood being very few, sometimes only one ; but 
these adults, if such they may be called, still continue to grow, 
becoming at each moult stouter and at the same time less galeate. 
I have several times seen examples of such adults just at the moulting 
stage, and in such cases the new skin was seen to be closely approxi - 
mated to the old one over the whole surface of the body, excepting the 
tip of the posterior spine and the anterior part of the head, where the 
new galea was considerably less acute than the old one (see Plate XIII. 
fig. 1, Loch Frisa). This reduction of the galea may continue until it 
disappears entirely and the creature has an absolutely rounded head. 
Not infrequently, however, the oldest Daphnias of a tow-netting 
show a reminiscence of the juvenile peak in the shape of a slight 
angularity on that part of the head (see Plate X. fig. 3, Loch Valtos). 
In the specimen just referred to from Loch Frisa the tip of the 
newly formed posterior spine did not reach up to the end of the old 
one, the new spine being shorter than the old one by about one-sixth 
