290 THE FRESH-WATER LOCHS OF SCOTLAND 
immersed in water, but can live equally well on land if their roots 
can have sufficient moisture. Another group of rather different nature 
contains animals which live among moss. They are only functional 
when surrounded by water, yet they do not die (or need not) when 
their habitat dries up. These animals are in various ways protected 
against desiccation, and though they are only active when moist, 
they can survive droughts. Most of these animals are so completely 
adapted for this intermittent life (they pass through a resurrection 
at every shower of rain) that they are often unable to survive pro- 
longed immersion if the water becomes in the least stagnant. The 
water of the larger lakes remains so fresh and well-aerated that such 
animals, casually introduced, it may be, live well and appear often 
to be permanently established in lakes. All but one or two species 
of the Bdelloid Rotifers and 1 ardio-rada, of which orders there are 
altogether nearly 100 species in the Scottish lakes, belong to this class 
of terrestrial animals. 
Only the shallow lochs, and sheltered bays in the larger lochs, 
have the marginal vegetation which supports the rich microfauna and 
microfloia. The exposed shores are generally bare and stony, and 
perfectly devoid of higher vegetation. The stones are usually 
covered with a coating of slime, which is shown by the microscope 
to consist of Diatoms. When the lochs fall lowest, usually about 
July or August, the dried Diatoms form a white zone. It is the only 
instance of Diatoms becoming conspicuous in the lakes, though rarely 
Asterionella or Tahellaria take a share in colouring the water. 
The Abyssal Fauna 
The abyssal fauna is very scanty, and, so far as the observations 
go, very uniform in different lochs. The data available are, however, 
insufficient. There is no abyssal flora. 
It is necessary to define the term ahyssaL as here used, and to make 
comparison with the values given to the term elsewhere. Forel 
defines the abyssal region in the Lake of Geneva as beginning at 
about 100 feet, or immediately beyond the limit of chlorophyllaceous 
vegetation. He divides it into three parts : (1) the supeiior zone, 
down to about 200 feet, or the limit of actinic action in summer ; 
(2) the inferior zone, from about 200 feet to the edge of the central 
plain ; (3) the zone of the central plain. In this abyssal region, 
approaching so near the surface and the shore, there is a con- 
siderable flora, consisting almost entirely of lower Algae, chiefly 
Diatoms, but including one Moss. The fauna comprises 79 species, 
established or erratic, the great majority of which are not peculiar 
to this region ; but a certain proportion of species, as indicated 
