THE VISION OF MOLLUSCS 



575 



portions of their shells outside their burrow, will depend much for 

 their survival on their ability to withdraw into safety before the 

 arrival of their many enemies. Pecten, with its elaborate eyes, is an 

 exception, perhaps because this animal may use sight to direct its 

 unusual activity as it " flies " on the water for considerable distances 

 by flapping its valves and expellmg water from the apertures near the 

 fringe. Even if this is not so, the experiments of Wenrich (1916), who 

 determined the smallest white card which produced a shell-closing 

 response in this scalloj), showed that the animal was extremely sensitive 

 to minimum changes in brightness. On the other hand, in abyssal or 

 underground Molluscs, visual organs tend to be less elaborate, and 

 vision takes a secondary or negligible place in the creature's activities. 



Snails have been trained to negotiate a T- or Y-maze (Garth and Mitchell, 

 1926 ; Fischel, 1931 ; Brandt, 1935), while a number of Molluscs demonstrate a 

 remarkable ability to seek their habitual homes from a dLstance.^ The mechanism 

 employed is unkno^\^l ; an association of several senses is possibly involved among 

 which touch probably figures largely and vision little if at all. 



CEPHALOPODS are visually in a very different class. There can be 

 little doubt that they use their eyes for the actual observation of 

 objects and in this respect, depending on vision rather than smell, they 

 are unique among Molluscs. Functionally their eyes are capable of a 

 considerable degree of pattern- vision, they have a good perception of 

 movement, and have adaptive and accommodative powers. They are 

 the only Invertebrates which exhibit pupillary reactions remotely 

 resembling those characteristic of Vertebrates (Magnus, 1902) ; these 

 reactions are most readily excited by yellow-green light of the same 

 spectral range which induces the most active phototactic responses. 

 Although many Cephalopods change their integumentary colour to 

 harmonize with their background by reflexes originating in the eyes,^ 

 Carl von Hess (1921-22) found no evidence to suggest that colour 

 vision is present ; and the positive claims made by Goldsmith (1917), 

 Bierens de Haan (1926), Tinbergen (1939) and Kiihn (1930-50) that, 

 as judged by behavioural experiments, they can differentiate hues are 

 open to serious criticism (Carter, 1948). 



The visual capacity of Octoinis has received a considerable amount 

 of attention by such writers as von Uexkiill (1905), Polimanti (1910), 

 Goldsmith (1917), ten Gate and ten Cate-Kazejewa (1938), and 

 particularly by Boycott and Young (1950-56) and Young (1956). The 

 standard lay-out of their experiments was to allow an octopus to attack 

 and eat a crab associated with a particular geometrical figure, but to 



* Chiton, Pelseneer, 1935 ; the limpets, Patella and others, Davis, 1885-95 ; Lloyd 

 Morgan, 1894 ; H. Fischer, 1898 ; Pieron, 1909 ; Thorpe, 1956 ; the littoral Pulmonate, 

 Onchidium, Arey and Crozier, 1918. 



8 p. 93. 



Pecten 



Octopus 



