572 



THE EYE IN EVOLUTION 



Planarian 



Leptoplana 



Lumbricus 



THE VISION OF WORMS 



As would be expected from the extreme primitiveness of their 

 ocular structures, the vision of worms is limited to an appreciation of 

 the presence or intensity of light associated with a light-shadow reflex 

 usually of a photo -negative type ; as we have seen, in some species a 

 directional localization may be possible. In the unsegmented 

 WORMS the simple photo -negative reaction is the only response. 

 Planarians, for example, are always found in dark places beneath 

 stones or the leaves of water plants, vigorously retreating from light 

 whenever they are exposed to it (Taliaferro, 1920). Some such response 

 to light still remains when the eyes have been removed, the animal 

 depending on hght-sensitive cells scattered over the surface of the body. 

 There is evidence, however, for the first time in the animal kingdom, 

 that the rigid phototactic response can be modified in a very crude 

 manner by training. The Polyclad, Lejptoplana, for example, is 

 quiescent in the dark and moves when illuminated, but contact of the 

 head -end with a solid object stops the forward movement. Hovey 

 (1929) found that by simultaneously illuminating the worm and 

 touching it so as to prevent it from creeping forward, the photo- 

 kinetic reaction was completely inhibited ; a similar conditioning 

 process to electric shocks was demonstrated in Planaria gonocephala 

 by Dilk (1937). After removal of the cerebral ganglion these modifica- 

 tions of the simple phototactic response cannot be elicited, so that this 

 structure is necessary for the development of this elementary learning 

 process. In assessing the importance of these reactions to light in the 

 life of the animal, however, it must be remembered that the general 

 behaviour of unsegmented worms is determined not so much by their 

 light-sense as by the more fully developed chemical sense and the 

 sense of touch which responds with great sensitiveness to the slightest 

 movement of the water in which they live or to objects with which 

 they come in contact. 



More work has been done on the light -sense of segmented worms, 

 jjarticularly upon the earthworm, Lu7nbricus } It will be remembered ^ 

 that in this animal the light-sensitive cells are concentrated mainly 

 at the two extremities. In very dim illumination (less than 0-00118 

 m.c, W. N. Hess, 1924) the animal is j)hoto-positive, and in ordinary 

 daylight illumination, photo-negative — it must avoid light since, in 

 fact, exposure to strong sunlight for one hour may cause paralysis, 

 for several hours, death. It follows that on emerging from its burrow 

 at any time except at night or in the dim twilight of morning or 



i Hoffmeister (1845), R. Hesse (1896^ W. N. Hess (1924), v. Buddenbrock (1930), 

 Sefe-dl (1933), Unteutsch (1937-38). 

 . 190. 



