PROTOCOCOACES. 35 
The commonest of British species. A variety has been described 
which differs only in being entirely destitute of bristles, Wecan con- 
firm Ralfs in his observation that the species of this genus frequently 
make their appearance in clear water that is kept in glasses or bottles 
and exposed to the light. He says that he has repeatedly noticed the 
appearance of 3. acutus var. obliquus in bottles containing Desmidiew, and 
sometimes its rapid increase so as to outnumber its companions. Speci- 
mens obtained in this manner, he adds, are frequently more or less 
distorted. In little aquaria the present species often becomes a nuisance 
from its profusion. 
Plate XIII. fig. 8. Cells magnified 400 diameters. 
Sub-Family 5. Hypropicrye. 
Individual cells oblong-cylindrical, united into a reticu- 
lated saccate ccenobium, all fertile, some producing macro- 
gonidia, which join themselves into a ccenobium within the 
mother-cell, others producing microgonidia, which are fur- 
nished with two vibratile cilia and a lateral red spot; these 
escape from the parent-cell, and, after a brief motile period, 
subside into protococcoid, thick-walled spores, 
Genus 26. HYDRODICTYON. Roth. (1800.) 
Characters the same as in the sub-family. 
“The genus Hydrodictyon comprises, as far as known, but w single 
species, which is common to North America and Europe. It grows in 
great abundance in the neighbourhood of Philadelphia, especially in the 
ditches and stagnant brick ponds in the low grounds below the city, 
known as the ‘Neck.’ There it very frequently forms floating masses 
several inches in thickness, and many feet in extent, so that with the aid 
of a rake it could be gathered by the bushel. When thus in mass the 
colour is very generally dingy and yellowish, although the fronds, when 
in active vegetative life, are mostly of a bright, beautiful green. The 
plant is in greatest profusion in June and July, after which time it 
gradually disappears, until in the autumn it is scarcely to be found, but 
early in the spring it reappears. The very young fronds are minute, 
oval, cylindrical, filmy-looking closed nets, with the meshes not appre- 
ciable to the eye; when growth takes place the fronds enlarge, until 
finally they form beautiful cylindrical nets, two to six inches in length, 
with their meshes very distinct, and their ends closed. In the bright 
sunlight, they, of course, by virtue of the life functions of their chlorophy], 
liberate oxygen, which, being free to the interior of the net, and its exit 
harred by the fine meshes, collects as a bubble in one end of the cylinder, 
and buoys it up, so that, the heavier ends sinking, the net is suspended, 
as it were, vertically in the water. I know of few things of the kind 
more beautiful than a jar of limpid water with masses of these little nets 
hanging from the surface like curtains of sheen in the bright sunlight. 
A few cells collected in the fall or early spring, if put into a preserving 
jar, and the water occasionally changed, will multiply, and in a little while 
become a source of frequent pleasure to the watcher. 
“ As the fronds increase in size they are always in some way or other 
broken up, so that, instead of being closed cylinders, they appear as 
simple open networks of less or greater extent. The extreme length to 
which the frond attains is, I think, very rarely over twelve inches, with 
meshes of about a third of an inch in length. The construction of the 
frond is always the same. It is composed of cylindrical cells united end 
