MARINE AND ESTUARINE DIATOMS. 
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surface, upon which are to be seen vast numbers of minute lines 
or striae , variously arranged and sometimes separated into 
distinct areas by smooth bars or empty spaces. Though usually 
free and independent they are often connected together in chains, 
sometimes side by side or end to end, in other instances by their 
alternate corners, thus forming either ribbands or zig-zag chains. 
Each individual, whether independent or connected with others, 
has a complete wall enclosing a cavity and is therefore a true 
cell, and the Diatoms belong to the group of the Unicellular Algae. 
Within the cell there is, of course, more or less protoplasm, the 
source of its vital activities, and with this various other sub- 
stances, including coloring matters, but while the other chloro- 
sperms, as their name indicates, contain much chlorophyll and 
are of a bright green tint, the Diatoms have relatively little of 
this substance and their color, instead of being green, is brownish 
yellow. But the most remarkable feature possessed by all the 
members of the group, is the power of secreting silica from the 
waters in which they dwell and depositing this in their tissues. 
Other plants, like certain grasses, do indeed separate silica and 
deposit it in their cells, thus acquiring increased strength, but 
in the Diatoms not only is the quantity comparatively much 
larger but it is so disposed as not only to give to the individual 
permanency of form and a considerable power of resistance but 
also to determine the wonderful and often very elaborate 
patterns with which the exterior of the cell is decorated. When 
we remember that the Diatoms stand very nearly at the base of 
the scale of vegetable life, it is truly wonderful to find them 
manifesting a constructive activity and a variety of architectural 
conceptions such as ain accomplished artist might well emulate. 
It is the formation of these siliceous skeletons, as transparent as 
crystal, which gives to Diatoms, as will be presently explained, 
their economic and geological importance. 
But what perhaps is still more wonderful is the power of 
independent movement possessed by many of the Diatoms, and 
which in some instances is truly marvellous. They may often 
be observed swimming across the field of view of the microscope 
