140 
BATHYMETRICAL SURVEY OF 
Diaptomus, Daphnia, and Bo%inina are occasionally found, but the 
species included in the preceding list are so much more common, that 
when the generic name only is mentioned it will be understood that 
the common species is referred to. 
This small association of animals and plants constitutes what may 
be called the lacustrine type of plankton. A not very dissimilar 
association is found in small ponds, but the species for the most part 
are different. The Diaptomus may be D. castor, the Daphnia D. pidex, 
the Bosmina B. cornuta ; Hotifers and Algae will be more abundant and 
varied, and there will probably be some Ostracodes. It might have 
been expected that the shallowest lochs would have had a plankton of 
the pond type, but it has been found that even the smallest lochs 
surveyed had the plankton distinctly lacustrine. A few nearly or quite 
stagnant lochans showed a slight approach to the pond type in the 
presence of Bosmina cornuta and Volvox and in the abundance of 
Rotifers and Algae. 
The remarkable variations of the loch trout, which have so much 
puzzled naturalists cannot be touched on here, but parallel cases are 
found among the smaller animals. Diaptomus gracilis varies remark- 
ably in colour, and is usually constant for each loch, and several other 
Entomostraca vary greatly in size and form; chief among these is 
Daphnia. The typical lacustrine form of this genus, which will be 
referred to as Daphnia lacustris, has an evenly rounded head with a 
depression on the line of the forehead marking off the brow from the 
beak. Where this depression is obliterated the head of the animal has 
a very different appearance, resembling that of a parrot. The form 
differing most from the typical Daphnia lacustris is that in which 
the head is produced upwards into a sort of peak or helmet. For 
convenience, this form will be referred to as Daphnia galeata, though 
it is doubtful if the points of difference are of specific value, and 
intermediate varieties are found. 
After Daphnia, the species which varies most is Bosmina 
obtusirostris . The typical lake-form has a short mucro at the posterior 
angle of the valves. It varies much in size and in colour, being usually 
hyaline, but sometimes purple, or rarely orange and purple. 
During its season Holopedium, from its large size, is very conspicuous 
in those lochs in which it occurs. It is frequently so abundant that it 
chokes up the nets in a short time, and makes it impossible to get a fair 
proportion of the other animals present. It appears in some lochs as 
early as May, and continues till August. 
Commonly a single organism, usually vegetable, will so increase in 
a loch as to form what the Germans call a Wasserblut.’’ The Algse 
Clathrocystis, Oscillaria, Botryococcus, Anabccna, and Volvox are among 
those which most frequently increase to this extent, but almost any of 
the smaller organisms, as Diatoms, Rotifers, or Protozoa, may on 
