CH. VIl] SNAILS AND SLUGS 215 



snails use these cords as a convenient and direct route to 

 the surface for respiratory purposes or for a quick return 

 to the depths when a fresh supply of air has been taken in. 

 A Limncsa in climbing up its cord often folds the foot 

 longitudinally so as to approximate the right and left 

 edges and enclose the cord in a temporary tube. This 

 manoeuvre brings the whole ventral surface of the foot in 

 contact with the cord. At the same time muscular 

 movements of the body are visible which strongly suggest 

 that the animal is pulling itself upwards. It is neverthe- 

 less most probable that in these relatively rapid ascents 

 and descents hydrostatic principles are chiefly concerned. 

 I have seen a Limncea in an aquarium ascend from a 

 depth of eight inches with a bubble of air projecting from 

 the respiratory aperture. On arrival at the surface the 

 animal rolled over so as to bring the aperture to the surface, 

 and the bubble burst with an audible explosion. The lips 

 of the aperture were then projected funnel-wise above the 

 surface for some seconds while fresh air poured in ; this 

 was repeated several times, the aperture being closed after 

 intake of air and lowered just beneath the surface. Having 

 obtained enough fresh air the Limncea descended again, 

 alighting, thanks to the guide rope, upon the exact spot 

 whence it had started. The rise and fall chiefly depend, as 

 I think, upon the amount of water displaced by the snail 

 and its contained air: the animal is a living Cartesian 

 diver. 



A similar power of forming threads 1 is possessed by 

 many slugs, especially when young, and enables them to 



1 Zykoff, Zool. Anz. 1889 ; Martins, ibid. 1878 ; Eimer, ibid. 1878 ; 

 Tyd. Quart. Journ. of Conch. 1878. 



