VII SHORT SIGHT IN SNAILS I 85 



Faludina 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,^ 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 distin- 

 guish 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 com- 

 partment, 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 Planorhis, are lovers of darkness, while 

 H. nemoralis, Succinea ;putris, and two species of Lim.naea 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 con- 

 clusive, but H. aspersa, A. empiricorum, several species of Limax, 

 and one Lininaea shunned or sought the light just as they had 

 done when their eyes were present. A few marine IMoUusca 

 {Littorina 'littorea, Trochus cinerarius, T. umbilicatus, Patella 

 mdgata) were also shown to be exceedingly sensitive to the impact 

 of a shadow, whether with or without their eyes. 



Blind and Eyeless Mollusca. — In a large number of marine 



1 Arch. Biol. xii. 1892, p. 57. 



