358 THE FUESH-WATER LOCHS OF SCOTLAND 
The importance of the class Chlorophyceae is much greater in 
fresh water than in salt. The whole of the order Conjugatae, includ- 
ing the unicellular Desmidiacese, is confined to fresh water, in which 
there are no more characteristic types than such as Spirogyra^ 
Zijgnema^ Cosinarkmi^ Staurastrum^ Micrasterias^ and XanthMmm. 
Other characteristic fresh-water Chlorophyceae belong to the genera 
Scenedesnius^ Pediastrum^ Oedogonhmi^ Cladophora^ and Vaucheria. 
Familiar marine forms are Ulva, Cmderpa, and a species of Clado- 
phora. 
The two groups Diatomaceae and Peridineae together furnish the 
main mass of the vegetable plankton in the sea, but while the Diatoms 
are also of some importance in fresh water, the Peridineae are 
represented by comparatively few forms. Among the latter, mention 
may be made of a cosmopolitan fresh-water type, in Ceratium 
hirundmella. 
The Myxophyceae and Bacteria are both more generally distributed 
in fresh water than in salt. Of the former, the Oscillatoriaceae are 
represented in the sea, and certain Bacteria are abundant in shallow 
water near the coast. Still, these two groups are more prominent in 
fresh water, both as regards the number of forms and the number 
of individuals, there being among the Myxophyceae several genera 
(Oscillatof'ia, Gomphospluvria, Clathrocystis^ Anahcena), species of which 
may appear in such quantities in lakes as to produce the phenomenon 
known as " water-bloom. 
We may now, as in the case of the animal kingdom, briefly gather 
together the most striking points in the distribution of fresh- and salt- 
water plants. In the sea we find no Dicotylae, Pteridophyta, 
Bryophyta, Characeae, or Conjugatae, and only comparatively few 
Monocotylae. In fresh water there are no groups containing aquatic 
plants which are quite unrepresented, but the Peridineae occur only to 
a limited extent, and the Phaeophyceas and Rhodophyceae in very 
small numbers. 
By our rather detailed examination of the organisms of fresh and 
salt waters, it has become clear that there is a very definite series of 
forms perfectly characteristic of the one medium or of the other. 
There are, however, some striking cases known, which would seem at 
first sight to entirely disprove this statement. TYiq Caspian Sea, in 
spite of its name, is in some regions, and particularly in the surface 
layers, less than one-fifth as salt ^ as the ocean, and thus may almost 
be considered a fresh-water basin. Yet the fauna includes many 
forms which we cannot but regard as typically marine. In addition 
to characteristic fresh-water animals {Sihirus^ Cyprinus^ Astacus)^ we 
^ Quinton, ojj. ciL, p. 215. 
