VII SHORT SIGHT IN SNAILS 1 85 



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 Pul- 

 monata 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 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^ Ario7i empirieorum, six species of 

 Limax^ and three of Planorhis^ 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 conclu- 

 sive, 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. umhilicatus, Patella 

 vulgata) 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. 



