214 SNAILS AND SLUGS [CH. VII 



tracks have been found on the glass roofs of greenhouses 

 from which snails have removed the whitening, and may 

 be seen on the bark of trees over which they have crawled. 

 These last are sinuous bands from which the microscopic 

 Algae, Pleurococcus 1 , (the " green " of the bark) have been 

 removed by the browsing snail. 



The slime trails both of slugs and snails show clearly 

 that these animals possess a good sense of direction and 

 locality. Each individual as a rule returns after its nocturnal 

 forays to its own selected retreat, whence it issues again 

 the following evening. 



Aquatic snails such as Limncea glide along by similar 

 means in search of food over the surface of water plants or 

 in an inverted position just below the surface of the water 

 with the sole uppermost 2 . In this position advantage is 

 taken of the physical properties of the " surface-film " to 

 procure a relatively firm basis upon which the mucus can 

 be deposited. 



Many fluviatile molluscs, such as Limncea*, Physa, 

 Planorbis, Cyclas, and others, possess a more rapid method 

 of locomotion in a vertical plane. This consists in the 

 power of forming threads of mucus of considerable length 

 either in an upward or downward direction. These fila- 

 ments may be attached to objects below the surface of the 

 water or to the "surface-film" itself. In the latter case the 

 upper end of the filament is expanded and slightly concave, 

 and acts as a float supporting the thread below it. The 



1 Feuille de Jeun. Natiiral. (3) Ann. 28. 



2 Grafin M. von Linden, Biol CentralbL xi. 1891. 



3 Warington, Ann. Mag. Nat. Hist. 1852. 



