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W. H. TALIAFERRO 
ORIENTATION TO LIGHT IN NORMAL SPECIMENS 
With the exception of a few forms like Bdelloura, all of the 
species of triclads whose reactions to light have been studied 
are negative. The earlier investigators, Loeb (93, ’94), Hesse 
(97), Parker and Barnett (’00), maintained that the species 
worked on do not orient to the direction of the rays of light to 
any marked degree, but simply tend to come to rest in the areas 
of least intensity. Later investigators, however, Walter (’07) 
and Mast (’10), found evidence of fairly precise orientation. — 
Orientation to a horizontal beam of light 
Planaria maculata moves over the substratum with an even, 
gliding motion. During this process the anterior tip of the head 
as well as the cephalic lobes are considerably elevated above the 
rest of the body. There is no continual pronounced side-to-side 
movement of the anterior end. A given specimen may, however, 
at irregular intervals raise the anterior end and wave it from 
side to side, but this reaction usually continues only for a brief 
interval, after which the animal resumes its normal gliding course. 
This ‘waving’ reaction is what Walter (’07, p. 49) has termed ‘wig- 
wagging movements.’ He considers ‘wigwagging movements’ as 
attempts ‘‘on the part of the worm to become adjusted to the 
stimuli acting upon it.’’ Another movement of this general 
character which I have designated as the ‘twisting reflex’ will 
be taken up later. 
A specimen which is illuminated by light from a single source 
moves fairly directly away from the source. If, after such a 
specimen has become oriented in a horizontal beam of light, it 
is illuminated laterally by a sudden change in the direction of 
the rays through 90°, it usually turns directly away from the 
source of light, without preliminary trial movements, as indicated 
in figures 4 and 5. By referring to figure 4, which is a camera- 
lucida drawing, it will be seen that, in this experiment, the di- 
rection of the rays was changed five times and that the planarian 
turned directly from the light each time without trial movements. 
Approximately ninety camera-lucida drawings representing the 
