538 ScHAKFF — On the Slugs of Ireland. 



In the former the wrinkles on the back, just behind the mantle, are very long, and 

 sharjjly keeled, whilst in A. subfuscus they are hardly half as long, and flat. 



Too much importance, however, should not be attached to the shape of the 

 wrinkles. Anyone who has kept this species in captivity can see for himself that, 

 by carefully sprinkling the box in which the specimens are confined with water, 

 the body-wrinkles after a while become more and more flattened out. A specimen 

 which may have had all its wrinkles standing out sharply from the body, in a dry 

 atmosphere, appears quite changed after it has been imprisoned in a damp tin box 

 for a couple of hours. There are naturalists who have manufactured new species 

 of Arion merely by the different shapes of the wrinkles ; but a little practical 

 observation shows how much they are worth. An adult of A. ater cannot be mis- 

 taken for A. suhfusciis, but a 3'oung one might, and indeed has been by most 

 writers. A rough and ready method of discriminating between the two species is 

 to give them a tap on the head. A. ater will almost immediately draw itself 

 together, and resting on its foot, the arched body will appear nearly equal in 

 length and breadth (Plate LVI., fig. 11). Another tap now will, in almost every 

 case, even in very young specimens, cause the animal to rock about from side to 

 side. This most peculiar motion, which is often continued for several minutes, has 

 never, to my knowledge, been observed in any other slug. A similar tap applied to 

 A. suhfuscus will merely cause the animal to shrink up, but it will never assume 

 the characteristic hunched position of A. ater, nor will any tapping produce the 

 swaying movement. 



The margin of the foot is very often of a colour different from the rest of the 

 body ; thus in brown specimens the margin may be brick-red. On the Continent 

 specimens of a similar brick-red tint covering the whole body are extremely 

 common, and in central Europe the large majority are of this colour. (This will 

 be referred to again in the Chapter on Colour.) I have never seen an adult brick- 

 red specimen in Ireland. 



The head and tentacles are, as a rule, of a dark violet colour. There is no 

 trace of a keel, the back being perfectly rounded. 



This species assumes much larger ^proportions on the Continent than it does in 

 Ireland. The largest I have seen near Dublin measured 90 mm. by 10 mm. On 

 the West Coast specimens of that length, but rather broader, are common. The 

 average size for a full-grown specimen on the East Coast, however, is 60 mm, 

 by 10 mm. 



The nature of the mucus varies in proportion to the severity of the stimulus. 

 As a rule colourless, it becomes orange-yellow when the animal is much irritated, 

 and sometimes I have seen it milky like that of Agriolimaz agrestis. 



Anatomy (Plate LVII., fig. 32). — Detailed accounts of the anatomy of this 

 species have been given by Lawson (18), Nunneley (31), and others. I have 



