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' (V), 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 



light, as it is nearly always present in those motile 

 green cells which show sensitiveness to light, and is 



IDC" 



FIG. 6 (Volvocacese). 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, 

 cl ; 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.) 



