l'RIICI lljOK OKI, AM). M.-X. Kl. 



Variation. Most of the ylrioiis arc of special inlcrrsl with regard to 

 their great variation, which comprises not only the external appearance but 

 even all the internal organs that have been subject to close examination, 

 /'. e. chiefly the reproductive organs, the jaw and the radula, I venture 

 to say that the variation of these organs is perhaps a .somewhat conspicuous 

 result of the investigatif)ns which are set forth in the present paper. The 

 particulars regarding variation of the said organs will be mentioned under 

 each species. Only the following facts need be pointed out here. 



The reproductive organs in several species may present such ex- 

 treme variations as to make difficult or impossible the determination by the 

 help of characters commonly designated as most constant to the species. 



The jaw in most cases is only slightly characteristic, except in Ariou 

 (iter and Arioii suhßtscits. In the other Norwegian species its varying 

 shape will be of little value for the determination. 



It is often impossible to distinguish the species by the radula, a conse- 

 quence of the wide range variation, especially when including the foreign 

 stocks. Yet the five Norwegian species after a little practice may be 

 distinguished from each other in the cases which I should designate as 

 typical, judging from Norwegian specimens. — The median teeth are often 

 slightly unsymmetrical, sometimes rather markedly so. Other abnormities 

 of the teeth are not uncommon. In some cases the alteration of the teeth 

 from the median line outwards takes place more suddenly than in others. 

 Reference may also be made to the circumstance that the radula of the 

 young is often slightly different from that of adults, in addition to the teeth 

 being smaller. 



Distribution. Within the family, some species of Arioji are found 

 farthest to the north. The habitat of the genus is especially Europe, where 

 it occurs from the Arctic Ocean to the Mediterranean. To the east, the 

 limit is doubtful, and after critical investigations Simroth (1901) concluded 

 that in Asia only one species, Arion Sibiriens Simr., is of certain occurrence. 

 Moreover the genus is found in North Africa and even if in some cases 

 it was introduced by man, in the Atlantic Isles (Greenland? Iceland, the 

 Faeroes, the Azores, Madeira, St. Helena). As far as South Africa, New- 

 Zealand and North America are concerned, its existence has been shown, 

 but is was undoubtedly introduced into the latter continent by man, and 

 perhaps the occurrence in South Africa and New-Zealand is due to the 

 same cause. 



Bionomics. Like other slugs the Arious need moisture, and hide 

 themselves when the air and the soil are too dry; they are chiefly to be 

 seen in rainy weather or during the night. The Norwegian species, especially 

 when young, are capable of lowering themselves by a mucous thread. This 

 faculty has been given special attention by Kew (1902). Their food chiefly 

 consists of plants, but they are not seldom seen to feed on carrion, and 

 even on excrements. A very interesting statement was made by KCxkel 



