51Q DESMIDS AND DIATOMS. 
The close analogy existing between the structure and 
mode of growth of the Desmidiacez and that of other un- 
doubted unicellular Algæ became evident with the rapid 
improvements in the instruments of observation. Not so, 
however, with the Diatomacez, for although the opinion 
is now almost universal among those most competent to 
decide the question, that they are truly vegetables, this 
conclusion was long opposed by able writers, and even 
now is by some authorities strenuously disputed. With 
the majority of observers I will assume their vegetable 
nature, and endeavor to describe the most important fea- 
tures in their structure and mode of growth. 
The Diatoms (from dia and temno, to cut in, two) are, 
like the Desmids, unicellular Alge, consisting essentially 
of two plates, concavo-conyex, Dai, hemisphere or 
depressed, connected together by a band or hoop called 
the “connecting membrane,” applied to and coinciding 
with the circumference of the valves, and enclosing within 
a coloring matter, which, unlike that of the Desmids and 
most true plants, is of a yellowish or orange-brown color. 
The form, structure, and marking of these valves are 
often very complex as well as beautiful, and are of vast 
importance in the classification of these organisms. As 
to form, the Diatoms present an infinite variety of size 
and outline. Mathematical curves of the most exquisite 
perfection, combinations which the designer would grasp 
with eagerness in the planning of his “modeli, surfaces 
adorned with the most unlimited profusion of style and 
ornamentation, are everywhere presented. They may 
conveniently be arranged under two great heads; first, 
of the forms more or less linear or eiii in outline, in- 
cluding most fresh-water species; and, secondly, those 
- Which are eet almost solely marine. This division is 
