SLUGS AND SNAILS 155 



Providence had left the naked snails without any defence 

 against their enemies, yet they have many means of escaping 

 danger. The slug contracts itself into a heap, and throws from 

 the pores of its body a thick slimy mucilage, which renders it- 

 difficult to make any impression upon its body, and is no doubt 

 extremely offensive to many of its brute enemies. The crystal 

 transparency of many of the marine species, which renders them 

 almost undistinguishable from the clear sea water, screens them 

 from numerous persecutions. Those that creep, find excellent 

 places of concealment in the crevices of rocks, or among the 

 branches of the madrepores; and the dorides, on contracting, cast 

 off parts of their mantle, which they leave in possession of their 

 hungry foe, while they themselves make their escape. 



However different the form of the shell may be, its use is 

 invariably the same, affording the soft-bodied animal a shield 

 or retreat against external injuries. In this respect it is not 

 uninteresting to remark that those species which inhabit the 

 coasts, and are most exposed to the rolling of the waves, have 

 thicker and stronger shells than those which live in greater 

 depths, and that the freshwater molluscs have generally a 

 much more delicate and fragile coat than those which live in 

 the ocean. 



The ianthinge, however who, unlike the generality of shell- 

 bearing gasteropods, pass a great part of their lives floating on 

 the water form a remarkable exception to this rule, as in 

 accordance with their mode of life their transparent shell is 

 extremely light. Their foot also is provided with a vesicular 

 organ which they are capable of inflating with air, and thus 

 buoyed up ( like little wanton boys that swim on bladders ' 

 they often appear in vast shoals upon the surface of the seas. 

 As soon as the winds ruffle the ocean, they immediately empty 

 their air-cells, and sink down into a more tranquil element, 

 and as a means of defence against their hungry foes they are 

 capable of pouring out a purple liquid which tinges the water 

 all around. 



It is an invariable rule that, the greater the necessity of pro- 

 tection, the better has nature provided for the want. Thus 

 most of the larger sea-snails, besides possessing a stone-hard 

 dwelling, are also furnished at the extremity of the foot with an 

 operculum, or calcareous lid, which fits like a door upon the en- 



