120 THE LIFE OF THE MOLLUSCA 



wearer less conspicuous in the shadow of vegetation. 

 The dun colour of the Desert Snail {Helix desertorum) , 

 like that of the other desert animals, harmonizes with 

 the prevailing tint of the habitat. The arboreal 

 A riophanta Dohertyi of Sumatra is of a delicate green 

 colour, and almost invisible amongst the foliage on 

 which it dwells. Many of the Slugs, by their colour- 

 ing and markings, are rendered inconspicuous in 

 their natural surroundings, such as Limax arborum 

 on trees, Geomalacus (the Kerry Slug) on lichen- 

 covered rocks. Certain of the Philippine genus 

 Helicostyla carry colour protection to a higher stage : 

 when dry their grey-brown periostracum, which is 

 full of air spaces, matches in colour the dead foliage 

 of the forests where these Snails have their abode ; 

 when the heavy showers, which are frequent, impart 

 a rich dark colour to the dead leaves, these air spaces 

 are filled up by the moisture, and their periostracum 

 then allows the underlying dark colour of the true 

 test to show through, so that the shell once more 

 harmonizes with its surroundings. On drying again 

 it speedily resumes its former hue. 



The reason for the coloration on some shells — 

 Cones, for instance — that are covered during life 

 with a thick periostracum is not readily explicable. 



Many of the marine shells, however, that are not 

 thus invested, are so marked as to be inconspicuous 

 in their proper habitat ; while others are coloured 

 to match the objects on which they live or prey. 



