52 THE EVOLUTIONIST AT LARGE. 



when attacked ; and if they only had a lid or 

 door to their houses they could shut them- 

 selves up hermetically, as periwinkles and 

 similar mollusks actually do. Other kinds, 

 like the pretty golden amber-snails which 

 frequent marshy places, have a body much 

 too big for its house, so that they cannot 

 possibly retire within their shells completely. 

 Then come a number of intermediate species, 

 each with progressively smaller and thinner 

 shells, till at length we reach the testacella, 

 which has only a sort of limpet-shaped shield 

 on his tail, so that he is generally recognised 

 as being the first of the slugs rather than the 

 last of the snails. You will not find a testa- 

 cella unless you particularly look for him, 

 for he seldom comes above ground, being a 

 most bloodthirsty subterraneous carnivore 

 who follows the burrows of earthworms as 

 savagely as a ferret tracks those of rabbits ; 

 but in all the southern and western counties 

 you may light upon stray specimens if you 

 search carefully in damp places under fallen 



