II.] INTRODUCTION. 



XXXV11 



duce them. Nearly a century ago, the experiment of 

 decapitating unfortunate slugs and snails was con- 

 ducted on a wholesale scale on the Continent, and every 

 philosopher was anxious to cut off a head. Even the 

 great Voltaire followed the universal fashion; and his 

 experiences were published in the " Questions sur 1'En- 

 cy elope" die." In these he mentions having operated on 

 twenty brown slugs and a dozen snails ; and he after- 

 wards records with great pride and satisfaction "mes 

 Limaces " and " mes Escargots " showing their budding 

 heads and horns, and doing as well as might be expected 

 under the circumstances. 



Phosphorescence. Although a great many animals, 

 from the highest order of fishes to the imperfectly orga- 

 nized Noctiluca miliaris, as well as several of the Tuni- 

 cata, emit or exhibit a phosphorescent light, I am not 

 aware that any of the Mollusca possess the same property, 

 except some kinds of Pholas ; and it is possible that the 

 phenomenon in their case may be owing to animalcula 

 which infest them or are found in their holes. The 

 eggs of a common slug (Arion hortensis) are said to be 

 luminous for the first fifteen days after they have been 

 laid. But both the nature and object of this common 

 phenomenon requires further investigation. 



Perforating powers. Many shell-fish, and especially 

 bivalves, burrow in sand or mud for protection against 

 their natural enemies ; but some of them excavate wood, 

 peat, or stone of various degrees of hardness, for the 

 same purpose. The process of burrowing is undoubtedly 

 performed by the foot of the Mollusk. It is exemplified 

 by the case of the common Cockle, which uses its tongue- 

 shaped and flexible foot in the same way as a gardener 

 uses his dibble, and, having thrust it into the sand and 

 expanded it, thus makes a hole large enough to contain 



