120 HUGH WATSON. 



sometimes have a slightly bluish or bliiish-green tinge, and 

 that this is probably due not to pigment, but to minute 

 particles in the outer layer of the skin which intercept tlie 

 blue rays. It is doubtful, however, whether this interference 

 effect Avould be as noticeable in living specimens. 



Apera possesses two chief kinds of pigment: the first is 

 dark and relatively stable, the second is lighter, very unstable, 

 and superposed upon the first. 



The dark pigment is usually brownish-grey, varying from 

 dull brown in typical examples of Apera gibbon si to dark 

 grey in A. purcelli. It does not spread uniformly over the 

 back and sides of the animal, but is more concentrated in 

 some parts than in others. In the first place the dark pigment 

 tends to be aggregated into small irregular patches, which 

 give the slug a mottled appearance. This tendency is least 

 apparent in Apera purcelli, and most marked in A. 

 dimidia, A. burnupi, and some varieties of A. gibbonsi. 

 In these animals the patches of colour are well defined, so 

 that the skin is conspicuously mottled. AVhen the dark 

 patches are examined through a strong lens they are seen to 

 -contain minute lighter specks, which are probably caused by 

 the dermal mucous glands. 



Secondly, the dark pigment generally shows a tendency to 

 become concentrated along the sides of the dermal grooves. 

 Thus in A. gibbonsi the oblique sub-lateral grooves are 

 usually rendered more conspicuous by the way in which the 

 patches of pigment tend to coalesce along their courses. 



Thirdly, the dark pigment is more abundant on some 

 regions of the body than on others. In Apera gibbonsi 

 it is concentrated dorsally, although a lighter band is gene- 

 rally left along the very centre of the back (PI. VIII, figs. 

 14-19). On the sides of the animal the mottling becomes 

 sparser, and it dies out before reaching the foot-fringe. 

 In the keeled species, as well as in A. dimidia and A. 

 purcelli, the dark pigment tends to be concentrated 

 laterally, so as to form an irregular longitudinal band on 

 each side of the bodv. Between the bands there is a lig-hter 



