ORIENTATION IN COLONIAL FORMS 147 



the process of locomotion and the reactions in both Pan- 

 dorina and Eudorina are in all essentials like those in Vol- 

 vox. They are negative in strong and positive in weak 

 illumination, but the degree of intensity in which they are 

 positive or negative varies greatly. They always swim 

 with the end containing the larger eye-spots ahead. They 

 usually rotate counter-clockwise on the long axis and pro- 

 ceed on a straight path. Only a very few colonies were 

 found to swim in a spiral course. If the light intensity is 

 decreased or increased with or without a change in the 

 direction of the rays there is no shock effect, nothing 

 resembling an avoiding reaction. If the general direction 

 of the rays is changed, positive specimens turn directly 

 toward the source of light without any preliminary move- 

 ments whatever, and negative specimens always turn in the 

 opposite direction. If exposed to light from two sources 

 so situated that the rays cross at right angles they swim 

 toward or from a point situated between the two sources. 

 The location of this point depends upon the relative inten- 

 sity of light from these sources. Orientation in these forms 

 takes place just as it does in Vol vox. 



a. Function of the eye-spots. — The eye-spots in both 

 Eudorina and Pandorina are located on the outer posterior 

 surface of the zooids just as in Vol vox. They have the 

 form of a segment of a sphere. The flat surface which is 

 slightly concave faces out. Those at the anterior end of 

 the colonies are much larger than those at the posterior, as 

 represented in Fig. 21. The former are nearly 2.5 ^u in sur- 

 face diameter, and 0.9 m thick; the latter are only approxi- 

 mately 0.6 fjL in diameter, but relatively thicker than the 

 former. They are reddish brown in color and stand out 

 boldly in strong illumination from below, showing that they 

 are comparatively opaque. In direct sunlight they become 

 luminous, giving off a greenish blue light, and as the colo- 

 nies rotate they sparkle and glitter, presenting a wonderfully 

 beautiful spectacle. After having been in direct .sunlight 

 for some time they are also luminous in diffused sunlight, 



