FRESH-WATER PLANKTON 
343 
from Loch Ba ; in Loch Frisa, the temperature being 59° '1 Fahr,, the 
oldest Daphnias had round heads, other adults had slight galeation, 
the middle-aged Daphnias were definitely galeate, and the very 
youngest had completely rounded heads. They were on the whole 
very like the North Uist or Lewis forms. 
The Orkney Islands were visited 15th to 27th August 1903, when 
Daphnias were abundant in all the lochs examined. An Orkney 
Daphnia is very distinct from any so far considered, being considerably 
larger, and the head being of a very characteristic shape. The young 
are slightly peaked, have long posterior spines, and, though some- 
what larger, they are in general shape much like the young of Lewis. 
The middle-aged Daphnia is stouter, the head is relatively larger and 
more galeate, and the posterior spine is already considerably reduced 
in length ; the adults are very large and stout, the posterior spine is 
reduced to a thickened stump, brown in colour, and the large head, 
which is slightly peaked, protrudes excessively inferior to the eye. 
The adult carries about nine summer eggs. Sometimes the reduction 
of the galea in the adult is complete ; in one loch. Loch Kirbister, 
galeation in the younger forms is but slight, whilst in Loch Harray 
there seems to be no attempt at galeation. In this latter case, however, 
the Daphnias were forming ephippia ; unlike those from the other 
Orkney lochs, the posterior spine showed no tendency to reduction, 
and on the whole there was a greater resemblance to the Shetland 
forms than to the typical Orkney Daphnia. 
Some of these Orkney lochs were visited again three years later 
(August 1906), and exactly the same type of Daphnia was found, 
so that this is evidently a permanent form, at any rate so far as 
the summer months are concerned. 
The total length of a typical adult from an Orkney loch is 
2*75 mm. (including the stumpy spine). These Daphnias are quite 
the largest I have seen ; the Shetland forms are longer, as the spine 
is not so much reduced, but otherwise they are not so bulky as the 
Orkney Daphnias. Mr T. Scott ^ has observed individuals as long 
as 3*4 mm. in Loch Oich, but these had a long galea and a moderate 
posterior spine. 
The Shetland Islands were visited 30th June to 8th August 1903, 
when Daphnia was abundant. In these lochs there was no trace 
whatever of galeation, and in all stages the rounded head was un- 
usually large ; they are quite distinct from the obtusifrons and 
microcephalic forms of Lewis. The young have posterior spines 
which are as long as the body, not including the head ; the posterior 
spine undergoes gradual reduction until in the adult it may become 
only one-eighth or one- tenth of the body -length. Such short spines 
1 Seventeenth Annual Report^ Fishery Board for Scotland, Part III. p. 196, 1899. 
