340 
THE FRESH- WATER LOCHS OF SCOTLAND 
the length of the latter ; this shortening of the spine, which at first 
is nearly as long as the body, and eventually may not be longer than 
one-sixth of the bodv-length — and, as we shall see, is reduced to a 
mere stump in the Orkneys — commences, I believe, at the early 
moults of the young and continues throughout life, being more 
pronounced, however, in the later stages of the animal's life. 
In the island of Lewis there are lakes of widely different depth 
and shape, and thei'e are corresponding differences in the temperature 
of these lakes ; nevertheless the Daphnia varied but slightly from 
lake to lake. In Loch Suainaval (24th July), which is of great 
depth and has a small limnetic area, the adults have just an indica- 
tion of a galea, while the young ones are decidedly galeate, but not 
much so. In Loch Langavat (16th Julyj, a large loch of some depth, 
Daphnia was rare, and the only specimens found were adults with 
protruding foreheads, but no galea. In Loch a' Chlachain (10th July), 
a shallow loch, the young are pronouncedly galeate, and the adults 
but slightly so. In Loch Dhomhnuill Bhig (14th August), a shallow 
loch, the adults are round-heads and the young ones galeated. In 
Loch Trealaval (7th August) the old ones liad round heads, the 
young ones were galeate, but not much so, and the youngest specimens 
were only very slightly galeate. In Loch Cro Criosdaig (4th August) 
all the Daphnias were slightly galeate. In these several forms the 
eye varies in size ; generally it is relatively small in the galeated 
Daphnias, and moderate-sized or large in the round-head forms. 
From this short sketch of the life-history of Daphnia lo7jgispina 
in the lakes of Lewis, it will be seen that Daphnia galeata^ s. str., is 
here to be regarded as a juvenile form, whereas elsewhere — on the 
continent of Europe, for instance — such is not the case, for there this 
galeated form, though not a permanent species, attains its maximum 
size, and produces epliippia without losing the galea. The conclusion 
I have arrived at is based simply on the facts just detailed, viz. that 
from 10th July up to 21st August, during which time tow-nettings 
were taken from more than twenty lochs, the Daphnias of any of these 
lochs were of all sizes and of all stages of galeation, and that generally 
the most galeate forms were quite without summer eggs, whilst the 
largest forms, which also were the egg-bearing forms, had only slight 
galeation or had completely rounded heads. Sven Ekman states that 
in the mountain lakes of North Sweden the spring Daphnias belong 
to the forma microcephala^ Sars ; later on appears the forma ohtusifrons^ 
Sars, and the summer generations are of the forma galcata^ Sars. It 
would appear, however, that in Lewis the differentiation into these 
several forms has only just commenced, and that, whilst there is a 
strong tendency towards galeation in the summer generations, this 
for the most part affects only the young, whilst the adults remain 
