FRESH-WATER ALG. 277 
now conclusively proved to be a fresh-water alga. 
This is the well-known Volvox globator, which, like 
the Protococcus nivalis, consists of a single cell, 
possessing the power both of nutrition and repro- 
duction. It is a rolling crystal sphere studded 
with emeralds ; or to use less poetical words, it is 
a symmetrically formed sphere composed of per- 
fectly colourless transparent membrane, with col- 
ourless watery contents, without any aperture, 
and studded over at equal distances with small 
green spots, in quantity ranging from 30 to 300. 
These green spots are identical with the ciliated 
zoospores of other alge. Owing to the associated 
movements of these zoospores, their cilia project- 
ing through the enclosing membrane, the whole 
full-grown plant moves freely. in the water with a 
graceful motion, sometimes gliding slowly across 
the field of the microscope, then stopping and re- 
volving, and then combining both rotation and 
progression. It reproduces itself in a remarkable 
manner, no less than three generations being pre- 
sent within the parent envelope at one time; 
daughter-cells springing from the zoospores, and 
known by their greener colour, and grand-daughter 
cells produced from the daughter-cells by segmen- 
tation. This exquisite organism is found in most 
open clear ponds, whose water is free from sewage 
and maintained at a uniform level all the year 
