174 



ARION ATER. 



Variation. — Scarcely any species is more variable in its colouring than 

 Ario7i ater, but nearly all the variations resolve themselves into two chief 

 lines, the red and the black, the presence, absence or varying proportions 

 of tliese 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 growth 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 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, Arion 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 growth, 

 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 

 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 

 and lower section, or the 

 slightly differing points 

 of attachment of the 

 retractors are chiefly re- 

 lied upon as establishing 

 at least two species, which are distinguished as Arion empirieorum and 

 A. (Iter, the former name being usually though not invariably allowed to 

 include what is generally known as Arion riifu^. 



1 Monog. i., p. 330. 



Fig. 204. 



Fig. 20n. 



Proximal end of the Reproductive organs of Arion rtifus 

 and A . ater according to Pollonera. 

 Fig. 20i. — Arion nt/its (L.), sensu stricto (after Pollonera). 

 Fig. 205. — Arion ater (L.), sensu stricto (after Pollonera). 

 ep. epiphallus ; sp. spermatheca ; f^'. free oviduct; r. genital 

 retractor ; t.at. lower atrium ; u.at. upper atrium. 



