1 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 segment 

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

 which forms a continuation of the retina. The main portion 

 of the lens lies within the posterior optic chamber, 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 purpose, the sense of smell in their case 

 largely taking the place of visual observation. Madame Jean- 

 nette 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 nodif- 

 erus, and watch it attentively. 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 

 undoubtedly 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. Cyclostoma elegans and Paludina 

 vivipara are comparatively long-sighted, perceiving objects at a 

 distance of 20 to 30 centimetres.^ The increased power oft 

 vision is due, in these two cases, to increased elaboration in thej 

 construction of the eye, Paludina possessing a large and almost* 



1 Ann. Mag. Nat. Hist. (2), xx. p. 336. 



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. 



