THE VISION OF MOLLUSCS 



577 



Natural History described how Octopus would insert a stone between the open 

 shells of a bivalve so that the soft mollusc could be devoured at leisure, an observa- 

 tion repeated by Jeannette Power (1857) to demonstrate the importance of 

 vision in the behaviour of this creature. In her ac^uarium, she wrote, an octopus 

 holding a fragment of rock in one of its arms, intently watched the lamellibranch, 

 Pinna, until it opened its valves. As soon as these were fully opened, she 

 reported that with incredible address and promptitude the octopus slipped the 

 stone between the valves so that they could not close again, and thereupon set 

 about devouring its victim.^ Pieron (1909) claimed that Octopods were able to 

 uncork a bottle \i\ order to obtain crabs seen through its glass walls ; and other 

 somewhat similar statements appear in the semi -scientific literature. In view, 

 however, of the apparent inability of the octopus to use a " tool," it may well 



Fig. 730. — The Hunting Capacity of Sepia. 



Within a tank the cuttle-fish is situated at X. In the tank is a circular 

 opacjue bucket and an opaque eiiamel plate. A prawn to which is attached a 

 long thread is placed at A within sight of the octopod. As soon as its attention 

 had been drawn to it and it liad begun to follow the jjrawn, it was pulled by 

 tlie thread to position B behind the opaque bucket. The octoj^od followed, 

 whereupon the prawn was pulled behind the opaque jilate to C, again out of 

 sight of its pursuer. The latter would follow around B and thereupon it was 

 allowed to devour its prey (Sanders and Young). 



be that such stories are fairy tales or that the incidents were determined rather 

 by chance than by jaurposive behaviour (Boycott, 1954). 



THE VISION OF ARTHROPODS 



ARTHROPODS are a phylum so large and amori^hoiis that a study 

 of the visual perceptions of the various types must be taken separately ; 

 this diversity in function follows from an equally marked diversity in 

 habit and is to be expected within a group which contains members 

 smaller than some Protozoa with great simplicity in organization, 

 and others (particularly Insects) which are rivalled in their visual 

 capacity and learning ability only by the higher Mammals. Apart 

 from Insects, however, relatively little is known of the visual 



1 A somewhat similar story was recorded by Leonardo da Vinci (Manuscript H 14) 

 who described how crabs inserted a stone or twig into the open shell of an oyster. 



S.O. — VOL. I. 37 



