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 
Ariophanta 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. 
