550 Dr. C. A. Kofoid on Platydorina. 



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 airangement is not apparent, but it seems 

 to be correlated with the pronounced polarity of the organism. 

 Platydorina is positively pliototactic. A miscellaneous plank- 

 ton collection placed in a window with southern exposure in 

 an aquarium 6 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 

 slight increase above a moderate illumination causing them 

 to leave the field of view with considerable rapidity. In one 

 case, where twenty-eight colonies were in the field when 

 placed under the microscope, only one of them remained after 

 an exposure of twenty-five seconds. A very slight decrease 

 in the amount of light would invariably insure their return to 

 the field with almost equal rapidity, the number increasing 

 as the intensity of the illumination was decreased. It may 

 be that the asymmetrical position and the somewhat unusual 

 arrangement of the stigmata are connected with the pro- 

 nounced phototaxis of this organism. At least the asym- 

 metrical position has a tendency to place the long axis of the 

 stigma parallel to the main axis of the colony, with the outer 

 end directed towards the source of light in negative, and away 

 from it in positive, phototaxis. 



The flagella are uniformly two in number for each cell, are 

 similar in the same cell and in different parts of the colony, 

 and are in the adult colony 20-25 fj, in length. From the 

 outer pole of the cell they pass through the matrix, leaving 

 the appearance of a tube-like structure in the gelatinous sub- 

 stance (fig. 4). When not in activity the flagella project 

 beyond the sheath in a position perpendicular to the surface 

 of the colony at the place of exit. As in other genera of this 

 family, the flagella persist after the division of the cell to 

 form the daughter colony, and even after the divisions are 

 completed still provide locomotion for the maternal organism. 

 In some instances the flagella could be seen passing through 

 the matrix toward that cell of the daughter colony which 

 bears the lai-gest eye-spot. 



The contractile vacuoles (fig. 4, c.v.) are two in number, 

 and are located in the peripheral layer of protoplasm near 

 the outer end of the cell. They lie in the outer part of the 

 knob-shaped mass of protoplasm upon either side of the place 



