128 LIGHT AND THE BEHAVIOR OF ORGANISMS 



than they are when the opposite side is exposed. They 

 always turn toward a given side, which, as far as can be 

 seen, is not structurally defined. There is no evidence indi- 

 cating differential response to localized stimulation. 



3. Trachelomonas 



No detailed observations on the light reactions of Trache- 

 lomonas have been recorded. The reactions of this organ- 

 ism are of interest to us here chiefly because it is very 

 nearly radially symmetrical and because it has a very promi- 

 nent eye-spot located very near the middle of the anterior 

 end, a location quite different from that in any other form 

 of which I know. 



Trachelomonas hispida, the species studied most care- 

 fully, is ellipsoidal in form, about 0.02 mm. long and 0.015 

 mm. wide. It is surrounded by a dark brown rough brittle 

 test of considerable relative thickness. A single large flagel- 

 lum, frequently three times as long as the body, projects 

 through a hole in this test at the anterior end. A relatively 

 large contractile vacuole appears to communicate with the 

 exterior through this hole. This suggests that the flagellum 

 may possibly extend into the contractile vacuole as it does 

 in Euglena viridis. The eye-spot is reddish brown in color 

 and has the general form of a thin curved disk (Fig. 16). 

 It is located between the contractile vacuole and the anterior 

 end and appears partially to surround a canal leading from 

 the former to the exterior. The eye-spot is very irregular 

 in outline and appears under an oil immersion lens to con- 

 sist of a number of small granules embedded in a homo- 

 geneous matrix. The granules project in the form of 

 marked knob-like elevations on the convex surface, making 

 it appear very rough. In most specimens similar granules 

 were found lying about loose in the neighborhood of the 

 eye-spot. The test is so nearly opaque in many specimens 

 of hispida that little can be seen through it, while in some 

 other species studied it is actually black, so that nothing 



