THE SIMPLEST FORMS OF LIFE 39 
of the cell is occupied by colorless protoplasm, and 
near the outside is a bright red pigment-mass — the 
*“‘eye-spot” (e), near which can generally be detected 
one or two pulsating or 
contractile vacuoles, 
such as frequently oc- 
cur in the lower uni- 
cellular animals and in 
the slime-moulds. 
If these free-swim- 
ming green cells are 
placed in a glass vessel 
full of water, and placed 
where they are more 
strongly illuminated 
from one side, as for 
example in a window, 
it will be found that 
very soon they collect 
on the lighted side, and, 
if they are present in 
large numbers, may be 
seen to form a green 
line close to the side 
where the light is 
strongest. There is rea- 
son to suppose that the 
red eye-spot is in some 
way connected with 
this sensitiveness to 
Fic. 6 (Volvocacee).— A, a plant of 
Pleodorina Californica, showing 
the ciliated cells of which it is com- 
posed ; the arrow shows the direction 
in which it moves; B, one of the 
smaller cells, much enlarged, showing 
the two long cilia, c, the eye-spot, e, 
the nucleus, n, the pyrenoid, p, im- 
bedded in the cup-shaped chloroplast, 
el; C, three stages in the division of 
one of the large cells; D, the egg; E, 
spermatozoid of Volvox; F, two ga- 
metes of Pandorina fusing together 
to form the zygote, or resting-spore. 
(Figs. B, C, after Shaw; D, E, after 
Overton; F after Pringsheim.) 
light, as it is nearly always present in those motile 
green cells which show sensitiveness to light, and is 
