I 84 VISUAL POWERS OF CEPHALOPODA chap. 



round the whole circle of the eye, and into which project the 

 front portion of the lens and the folds of the iris. Throughout 

 its whole extent, the anterior optic chamber is lined by the 

 integument, the portion of which on the inner side is the 

 choroid. The lens is divided into an outer and inner seg- 

 ment by a thin membrane, and is supported by the ciliary 

 body which forms a continuation of the retina. The main 

 portion of the lens lies within the posterior optic cham- 

 ber, at the back and sides of which is found the retina 

 (Grenacher). 



There can be no doubt that the Cephalopoda use their eyes to 

 observe, but there is nothing to show that any other Mollusca 

 use their eyes for this pm^pose, the sense of smell in their case 

 largely taking the place of visual observation. Madame Jeannette 

 Power once saw ^ the Octopus in her aquarium holding a fragment 

 of rock in one of its arms, and watching a Pinna which was 

 opening its valves. As soon as they were perfectly open, the 

 Poulpe, with incredible address and promptitude, placed the stone 

 between the valves, preventing the Pinna from closing again, 

 upon which it set about devouring its victim. The next day 

 the Poulpe was seen, after crushing some Tellina, to stretch 

 himself down close by a Triton nodiferus, and watch it atten- 

 tively. After four hours the Triton emerged from its shell, 

 when the Octopus sprang upon it, and surrounded it with its 

 arms. 



Powers of Vision in Land Mollusca. — The Helicidae are un- 

 doubtedly very short-sighted. Seldom emerging from their retreats 

 except in twilight and darkness, they are naturally myopic, and 

 see better in a subdued than in a bright light. Experiment has 

 shown that a Helix can perceive an object better at 6 centimetres 

 distance in a weak light than at 4 or 5 millimetres in a strong 

 one. Gyclostoma elegans and Paludina vivipara are comparatively 

 long-sighted, perceiving objects at a distance of 20 to 30 

 centimetres.^ The increased power of vision is due, in these two 

 cases, to increased elaboration in the construction of the eye, 



1 Ann. Mag. Nat. Hist. (2), xx. p. 386. 



2 V. Willem {Arch. Biol, ut infr.) denies this, and declares that Cyclostoma is 

 only very sensitive to movements. The present writer has often approached, with 

 the greatest care, a crawling Cyclostoma, but it always withdrew into its shell or fell 

 to the ground when approached within about 10 or 12 inches. 



