INFUSORIA. 
53 
a. Tubes separate. 
a. Frustules strait,. Nauneraa,* 
b. “ curved,. Gloenema. 
b. Tubes joined. 
a. Fasciculate,. Schizonema.* 
b. Branching,. Micromega. 
The genera marked thus (*) have been detected in the United States. 
Ehrenberg remarks of this family, that “ the organization is difficult to recognize, in con¬ 
sequence of the hardness and refraction of the carapace. None have yet been found with 
calcareous coverings, but they are either hard and siliceous, (sometimes containing a little 
iron,) or membranaceous without silica. The differences observed in the forms of the cara¬ 
pace have been made use of in classifying them (see preceding table). In several genera, 
are found internal hyaline vesicles of variable form; these are colorless, and resemble the 
stomachs of the Polygastrica, and in recent experiments they have been colored by means 
of indigo. The female organs consist of colored or colorless granules, forming two or four 
groups, which are placed near the middle of the body, as in Navicula, Cocconema, Naunema, 
&c. These eggs are frequently divided into several globular groups, which finally unite in a 
cruciform manner (Achnanthes), or which become confounded together before being emitted 
(Gaillonella, Pyxidicula, Isthmia, &c.); at other times they appear under the form of a tube 
enveloping the stomachs and other organs (Xanthidium, Euastrum, Micrasterias). The genera 
Micrasterias, Arthrodesmus, Tessararthra and Xanthidium, have organs which may be com¬ 
pared to seminal vesicles. Spontaneous division produces much variety in the forms of the 
polypidoms. It sometimes takes place longitudinally, sometimes transversely.” 
The living species of this family may be found in almost every situation where water oc¬ 
curs upon the surface of the earth. Some genera are exclusively marine, others are exclu¬ 
sively fluviatile; while some genera, as Navicula, Gomphonema, &c. include both salt and 
fresh water species. The marine species may often be found in great quantities among the 
filiform algae, which they often invest completely with their crystalline carapaces. 
The fluviatile species may be found in every pond, stream, rivulet, bog, or pool, either 
nestling among conferva, parasitic on aquatic plants, or living in the sedimentary matter at 
the bottom. They often occur in such vast quantities as to cover hundreds of square yards, 
to which they give a peculiar color : green, yellowish or ferruginous, according to the peculiar 
internal coloring matter of the individuals. Most of the species are exceedingly minute, many 
are entirely invisible to the naked eye ; others, however, are quite perceptible without the aid 
of the microscope. Notwithstanding their extreme minuteness, it is evident, from their vast 
abundance, that they have some important offices to perform in the economy of nature; and 
like the coral insect, although the individuals are minute, the result of their united labors is 
on a scale by no means insignificant. 
Few organic bodies exceed in beauty the symmetrical, elegantly sculptured forms of many 
of the species. Their beauty, the singular phenomena they present, and the interest they 
