SLUGS AND SNAILS. 53 



leaves. Even in testacellse, however, the 

 small shell is still external. In this yellow 

 slug here, on the contrary, it does not show 

 itself at all, but is buried under the closely 

 wrinkled skin of the glossy mantle. It has 

 become a mere saucer, with no more sym- 

 metry or regularity than an oyster-shell. 

 Among the various kinds of slugs, you may 

 watch this relic or rudiment gradually dwind- 

 ling further and further towards annihilation ; 

 till finally, in the great fat black slugs which 

 appear so plentifully on the roads after 

 summer showers, it is represented only by a 

 few rough calcareous grains, scattered up and 

 down through the mantle ; and sometimes 

 even these are wanting. The organs which 

 used to secrete the shell in their remote 

 ancestors have either ceased to work alto- 

 gether or are reduced to performing a useless 

 office by mere organic routine. 



The reason why some mollusks have thus 

 lost their shells is clear enough. Shells are 

 of two kinds, calcareous and horny. Both 



