428 Illinois State Laboratory of Natural History. 



There is but a single, cup-shaped chromatophore (Fig. 4, 

 chr.), which is inclosed within the pellicle above noted and 

 itself contains the knob-shaped protoplasmic mass. It is of a 

 brilliant chlorophyll-green color, and contains numerous 

 small granules of irregular and somewhat angular outline. 

 Towards the inner end of the cell, imbedded in the thickest 

 part of the chromatophore, there is a single spherical pyrenoid, 

 having a diameter of 4-6 /v. 



The stigma, or eye-spot, seen from above is circular in 

 outline, but in lateral view has the form shown in Fig. 4, s. 

 The slightly convex outer surface appears to project some- 

 what beyond the rounded contour of the cell. The color is 

 usually a bright reddish brown, often brightest in the anterior 

 and marginal cells and rarely entirely faded in the posterior 

 ones. The stigma is a homogeneous body, showing no trace 

 of structure beyond the well-defined contour line, which is 

 best seen in fading and moribund cells. It is normally 

 present in all cells of the colony, and may readily be demon- 

 strated by full illumination. The position of the stigmata in 

 the cells is somewhat unusual, and is significant of the pro- 

 nounced polarity of the organism. The customary position 

 in other genera is adjacent to the bases of the flagella. In 

 Platydorina, however, the location of the stigma is not con- 

 stant with respect to the flagella, but seems rather to bear a 

 definite relation to the form of the colony, since it lies towards 

 the peripheral and posterior region of the cell (Fig. 1), while 

 the flagella are centrally located and project outward in the 

 usual manner. This relation appears not only in the mar- 

 ginal regions but also in the central. The physiological 

 significance of this arrangement is not apparent, but it seems 

 to be correlated with the pronounced polarity of the organism. 

 Platydorina is positively phototactic. A miscellaneous plank- 

 ton collection placed in a window with southern exposure in 

 an aquarium six inches in diameter was, after ten minutes, 

 quite barren of Platydorina except along the margin towards 

 the window. On the other hand, this species avoids bright 

 light. This was very evident in collections fresh from the 

 field when examined under low power (75 diameters), a very 



