174 ARION ATER. 
Variation.—Scarcely any species is more variable in its colouring than 
Arion ater, but nearly all the variations resolve themselves into two chief 
lines, the red and the black, the presence, absence or varying proportions 
of these constituents determining the tints ; their total absence causing the 
whitish or greenish varieties. 
The red pigment, which is said by Simroth to be a warning colour,’ 
resides in the dermal mucous cells, and is developed by warmth, a warmer 
or milder temperature during the erowth period increasing the proportion 
of rufous individuals, or intensifying and enriching their tint. When the 
colour glands are but feebly developed, it gives rise to the yellow tint and 
the intermediate shades. This colouring i is, however, very unstable and 
also in great part due to the mucosity, as when this is quite removed the 
animals often appear of an uniform grey or brown. 
The black pigment resides in the cellules of the integument, and its 
predominating development is in a large measure due to cold or moisture, 
as the dark varieties are found most numerously in cold or mountainous 
countries, Eimer especially remarking upon the predominance of the dark 
varieties at high altitudes on the mountains and also upon their greater 
prevalence upon the plains during wet seasons. 
In this country, also, Avon ater is usually dark coloured or black, this 
sombre colouring is, however, not invariably that of the youthful stages, 
but is usually an acquired colour, the result of changes during erowth, 
and though unicoloration is doubtless the ultimate or final stage of pig- 
mentation, the shade or hue is probably in great part dependent upon and 
modified by the conditions of the environment with which the coloration 
of the body tends to harmonize. 
Some of the more severely critical of modern authors variously divide 
Arion ater into two, three, or more species, influenced by trifling external 
differences or by slight 
inequalities in the degree rat \ " 
of development of “the 
various organs of the 
reproductive system, the 
modifications of which 
are due in a large degree 
to individual variation 
or to the stage of sexual 
maturity attained by the 
animals examined. ‘The 
differences in the amount 
of enlargement of the 
atrium or vestibule and 
its more or less apparent 
division into an upper Fic. 204. Fic. 205. 
and lower section, or the Proximal sud = Be Reproductive Greiasiot a rufus 
slightly differing points Fic. 204.— void, Sere Mea stricto (after Pollonera). 
of attachment of the Fic. 205.—A7rion ater (L.), sensu stricto (after Pollonera). 
retractors are chiefly re- ep. epiphallus; sf. spermatheca ; ov. free oviduct; 7 genital 
retractor ; Z.a¢. lower atrium; #.a¢. upper atrium. 
lied upon as establishing 
at least two species, which are distinguished as Arion empiricorum and 
A. ater, the former name being usually though not invariably allowed to 
include what is generally know! nas Arion rufus. 
1 Monog. i., p. 330. 
