574 



THE EYE IN EVOLUTION 



to become something that is not rigid and entirely automatic ; it can be 

 modified by experience and training, while vision, although still a 

 secondary sense, apparently becomes endowed with some degree of 

 awareness and meaning. 



Snail 



Slug 



Avicula 



Anodonta 



Mya 



THE VISION OF MOLLUSCS 



From the functional point of view in most Molluscs vision is 

 secondary to the olfactory or tactile sense ; this would be expected in 

 view of the primitive structure of the eyes of most types for, with the 

 exception of Cephalopods, they are rarely capable of detailed visual 

 resolution. It has been contended that land Molluscs (snails, slugs, etc.) 

 which seldom emerge except in twilight and retract their eyes within 

 their tentacles on exposure to bright light, are blind (Yung, 1913) (Fig. 

 188). A directional appreciation of light is possible, however, and quick 

 movements can be readily detected (Fob, 1932 ; Grindley, 1937) ; but 

 there is no evidence of the appreciation of colour (Mundhenke, 1955). 

 These animals, however, are highly myopic and experiment has shown 

 that objects can rarely be appreciated more than a few centimetres 

 away, although farther in subdued than in bright light (Willem, 1892). 

 Vision does not thus appear to dominate behaviour. On the other 

 hand, Gastropods are extremely sensitive to the slightest movement 

 of the air or any jarring of the surface on which they crawl, while their 

 sense of smell is so acute as to dominate most of their behaviour : food, 

 for example, is sought almost by scent alone. 



In littoral lamellibranch Molluscs it would seem probable that 

 vision is generally limited to the appreciation of light and shadow, but 

 this appreciation may be unusually acute. Whether the ocelli are 

 situated on the siphon or the mantle-edge the slightest shadow often 

 induces a response. Thus Patten (1886) found that in the Noah's-ark 

 shell. Area, the mantle contracted and the valves closed quickly if the 

 faint shadow of a hand or a pencil fell upon them. It is interesting 

 that sensitivity- does not always vary with the elaboration of the 

 structure of the eye, for the same observer found that an even more 

 sensitive response was given by Avicula which is provided with only a 

 few ill-developed ocelli; even the eyeless mussel, Anodonta, reacts to 

 a passing shadow owing to its dermal sensitivity to light (Braun and 

 Faust, 1954).! The rapidity with which oysters close their shells 

 on the passing of the shadow of a man or a boat is well known. 

 A similar sensitivity to passing shadows characterizes the ocelli in the 

 siphons of littoral Lamellibranchs (Hecht, 1919 ; Koller and 

 V. Studnitz, 1934, in Mya) ; and it is obvious that such types which live 

 between the tide-marks and protrude their siphons and occasionally 



1 p. 114. 



