22 ARIONID. 
Usually no shell is to be found, but now and then 
there are a few calcareous granules, and in rare instances 
in chalky districts these granules form a solid lump of 
considerable size. The foot-sole is sometimes bordered 
with black or brown, according to the colour of the animal. 
It seems to prefer decaying vegetation to fresh. I have 
several times found it engaged in acts of cannibalism, and 
I once found a colony of the beautiful var. albolateralis 
feeding upon excrement. These habits are ‘not nice,” 
and I have never had the courage to try this species 
as an article of diet. 
The very young are pale yellowish white. The tentacles 
first begin to turn bluish, then black, and then the back, 
the colour gradually spreading down to the fringe. These 
changes do not always vary directly as the size of the 
animal—perfectly black individuals sometimes occurring 
only a third of the adult size. 
Very little is known, as yet, of the causes of the coloura- 
ation of slugs. Differences of food, geological formation 
of habitat, inheritance and protective mimicry may all or 
some of them combine to produce the various striking 
and beautiful forms we meet with. 
This is a fascinating though most difficult problem, but 
one which can only be lightly touched on here. 
Dr. Scharff (“Slugs of Ireland,” page 554) says :—‘“ I 
think that the colours of slugs in Ireland are at all ages, 
as a rule, protective” (and if in Ireland, why not every- 
