THE VISION OF WORMS 573 



evening, either end will at once retract into the safety of its retreat. 

 Its more complex reactions to light when travelling on the ground 

 have already been [described.^ A similarly high degree of sensitivity is 

 seen among certain polychaete worms, particularly the tubiculous 

 types. In these the light-and-shadow reflex is very marked ; so 

 sensitive are they to light that Andrews (1891) found that if a hand 

 were moved in the air at a distance of a metre from the water containing 

 the animals, they withdrew themselves into their tubes as soon as the 

 shadow fell upon them. 



In segmented worms, however, the potentialities of habituation 

 and learning have evolved to a considerably greater extent ; that 

 earthworms, indeed, have a modicum of intelligence was known to 

 Darwin (1881) who noted the deft way in which by trial-and-error, 

 profiting by previous experience, they transported leaves of various 

 types to their burrow or collected little stones to guard its entrance.^ 

 Again, Hydroides, if collected from shallow water, reacts promptly to 

 shadows in the aquarium, but individuals collected from deep water 

 remain inactive presumably from lack of experience in a shadowless 

 environment ; a like passivity is rapidly assumed by reactive specimens 

 from shallow water if they are isolated from shadows for some time in 

 the laboratory (A. W. Yerkes, 1906 ; Hargitt, 1906-9). Similar 

 habituations to light-and-shadow stimuli have been found also in 

 polychaete worms (Bohn, 1902) and leeches (Gee, 1913). Moreover, in 

 these species the normal response can be varied by conditioning. Thus 

 the polychaete. Nereis, if presented with food together with a sudden 

 increase or decrease in illumination, can be trained after only six trials 

 to respond to the change in illumination alone whether it is positive 

 or negative (Copeland, 1930), while by a similar association with tasty 

 food or other stimuli a reversal of the usual reaction to light can be 

 induced in a number of worms such as Hydroides (A. W. Yerkes, 1906), 

 Nereis (Copeland and Brown, 1934), Lumbricuhis (Raabe, 1939) Nereis 



and Lumbricus (Wherry and Sanders, 1941). Finally, several 

 Oligochaetes and Polychaetes have shown a considerable ability to 

 learn the correct turning in a simple T- or Y-maze ; propelled forwards 

 by illumination of the hind region, rewarded by a warm dark cell or 

 punished by an electric shock or an unpalatable salt solution, they can 

 after many trials (up to 200) be taught to turn in the required direction, 

 a capacity unimpaired by excision of the supra-oesophageal ganglion.^ 



In worms, therefore, in which a ganglionated nervous system first 

 appears, for the first time in evolution the response to light has been shown 



1 p. 53. 



2 See also Malek, 1927. 



3 In Oligochsetes : AUolobophora (R. M. Yerkes, 1912), Eisenia and Lumbricus 

 (Heck, 1920), Heliodrilus (Swartz, 1929) ; in the polychgete, Nereis (Copeland, 1930 ; 

 Fischel, 1933 ; Copeland and Brown, 1934). 



