FRESH-WATER ALGE. 275 
and not depressions as in these plants. They are 
usually of a very deep vivid green colour, owing 
to the great quantity of chlorophyll which they 
contain. Individually their forms are oval, cres- 
cent-like, cylindrical, oblong, or with variously 
shaped rays or lobes giving them more or less a 
stellate appearance; but in their social state, 
when aggregated together, they are often arranged 
in linear series, collected into faggot-like bundles, 
or in elegant star-like groups, enclosed in a com- 
mon, very transparent gelatinous coat. As a class 
they are characterized by their bilateral sym- 
metry. Each frustule is in reality a single cell, 
which has a tendency to divide into two valves or 
segments; and this tendency is indicated by a 
constriction which is more or less deep in different 
genera. Owing to their power of locomotion, 
Ehrenberg and his followers regarded Desmids as 
of: animal origin; but this property, as we have 
seen, belongs to a large number of the lowest 
vegetable organisms. Their plant nature is now 
conclusively established. When mixed with mud, 
they are able to make their way to the sur- 
face ; and when contained in a bottle they begin 
to congregate on the side that is most exposed to 
the light. In some instances they have even been 
known to retire beneath the surface of the mud of 
ponds before it dries up, after the water has been 
