=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.[287] The increased power of vision is due, in these two cases, to increased elaboration in the construction of the eye, _Paludina_ possessing a large and almost spherical lens, to which the vitreous humour closely adheres, while in _Cyclostoma_ the lens is remarkably hard, and the aqueous humour very abundant. According to V. Willem,[288] the Pulmonata are very sensitive to the slightest movement of the air or jarring of the surface on which they crawl, but are so short-sighted as only to perceive a confused image of a large object at about 1 cm., and to distinguish the form of objects at not more than 1 or 2 mm. The senses of touch and smell are far more active than that of sight. A bean-pod enclosed in a narrow glass case and placed before a hungry snail was not noticed, but when taken out of the case and placed 8 cm. behind the snail, the latter at once turned towards it to devour it. Some interesting experiments were conducted by the same author with the view of ascertaining whether snails avoid or court the light. He placed a number of species in different wooden boxes, which were divided into a light and a dark compartment, having previously well soaked the boxes in water to secure a humid atmosphere and surface, and so induce the snails to move about. The result showed that nearly all species have a marked predilection one way or the other, but not all in the same way. _Helix aspersa_, _Arion empiricorum_, six species of _Limax_, and three of _Planorbis_, are lovers of darkness, while _H. nemoralis_, _Succinea putris_, and two species of _Limnaea_ are lovers of light. _Physa fontinalis_ stands alone in being quite indifferent. M. Willem endeavoured further to discover whether any of the Mollusca possessed ‘dermatoptic perception,’ or the faculty of perceiving variation of light by means of the skin alone. He accordingly repeated the above-mentioned experiments, having previously extirpated the eyes in all cases. The result was remarkable. In a few instances the experiment was not conclusive, but _H. aspersa_, _A. empiricorum_, several species of _Limax_, and one _Limnaea_ shunned or sought the light just as they had done when their eyes were present. A few marine Mollusca (_Littorina littorea_, _Trochus cinerarius_, _T. umbilicatus_, _Patella vulgata_) were also shown to be exceedingly sensitive to the impact of a shadow, whether with or without their eyes.