DESMIDS AND DIATOMS. 507 
solely to fresh water, where their delicate green cells, 
forming mucous tufts or films on the surface of boggy 
pools, or coating the stems or fronds of aquatic plants, 
often multiply in sufficient numbers to impart a pale 
green tint to the water. Only the most quiet and the 
purest water seem favorable to their growth. Running 
streams, brackish inlets, the turbid waters so productive 
of the minuter forms of animal life, seem wholly unfitted 
for these delicate organisms. Asa general rule they are 
much less numerous than the Diatoms, the latter far 
exceeding them in families, genera, and species, as well 
as in the number of individuals. Their outline is very 
varied, but generally characterized by a great number of 
incisions of greater or less depth, which seem constantly 
tending to divide the original cell into a number of 
smaller ones. Many of them have in general a circu- 
lar outline, but still marked with deep constrictions 
(as Micrasterias, Pl. 13, fig. 4), others are lengthened 
and sigmoid (Closterium, fig. 5), recalling some genera 
among the Diatomacese; others, again, are compound 
or concatenate ; and yet others, like the Diatoms, form 
long and waving graceful filaments. A tendency also 
towards outward development, as shown in the nume- 
rous projecting arms which are so prominent on many 
Species (figs. 1, 2, 6), is also a noticeable feature in their 
structure, and aids greatly in the determination of their 
Specific characters and their relations to other families. 
The Desmids consist, as I have said, essentially of a 
single cell. This cell has the usual number of external 
Coatings, a membrane of firm though flexible consistency, 
often containing particles of silica, and showing the affini- 
ties between this family and the Diatomacez ; a second 
Coating or “primordial membrane” included within the 
