On Platydorina. 429 



slight increase above a moderate illumination causing them 

 to leave the fiekl 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 pi 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 

 substance (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 organ- 

 ism. In some instances the flagella could be seen passing 

 through the matrix toward that cell of the daughter colony 

 which bears the largest 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 of origin of the flagella, being somewhat widely sep- 

 arated. At diastole the vacuoles of an adult colony have a 

 diameter of 1.5-2 /^. The contraction is rhythmical, and the 

 two vacuoles usually alternate at regular and equal intervals. 

 At a temperature in the laboratory of 20° C. each vacuole 



