THE SLUG AND ITS ALLIES 161 



Slugs belong to the group of air-breathing, land-inhabit- 

 ing gastropods, or Pulmonata. 1 They may be found in 

 the spring, summer, and autumn, under wet, decaying 

 pieces of wood, under stones or fruit, in the grass, on the 

 shady side of fences ; in a word, in moist, dark situations. 

 They especially shun sand, ashes, and sawdust, because 

 these substances tend to dry up the mucus which they 

 secrete over their body to retain its internal fluids. Slugs 

 are nocturnal (Fig. 122), hence they have the reputa- 

 tion of being rather rare. * During the winter they live 

 in the ground encased in their own slirne, but some species 

 frequent greenhouses in cold weather, and in consequence 

 of this habit may remain active all the year round. 



The food of slugs consists chiefly of the green leaves of 

 succulent plants, and also of ripe fruit, such as apples. 

 Our largest slug, Limax 2 maximusf is easily maintained 



fti. Gills in front of heart ; mantle-complex on 

 anterior side of intestinal sac ; operculum 

 constantly present ..... Prosobranchiata 



6 2 - Gills behind heart ; if shelled, without oper- 

 culum ...... Opisthobranchiata 



Key to the principal families of the American Pulmonata : 

 a\. Eyes at apex of (usually retractile) tentacles. 



61. With external shell ; 4 tentacles . . . Helicidce 



(Ex. Helix) 

 l) Z . Without external shell ..... Limacidce, 



(Ex. Umax) 



2 . Eyes on inner side or at base of the (non- 

 retractile) tentacles. 

 61. Thick shell, with thick,- often-toothed outer 



edge Auriculidce 



(Ex. Melampus) 



62- Thin shell, with sharp margin \ . . Limnceitlce 



(Ex. Physa) 

 1 Provided with lungs ; from pulmo, lung. 



, naked snail. 3 Largest. 



