CH. Ill] ARION AND TESTACELLA. 145 



to them without too much reason. Animal agencies for 

 the dispersal of both land and fresh water shells for long 

 distances across the sea are practically limited to birds ; 

 and as has been already mentioned this method of trans- 

 port must be rare as it has been so seldom observed. The 

 dispersal of slugs is of course harder to understand than 

 that of snails, as they have no protective shell. And yet 

 they are found in such remote islands as New Caledonia, 

 New Zealand, the Auckland Islands and some of the 

 Mascarene Islands. Madeira and the Azores have Arion, 

 which Mr Cockerell says 1 "has some appearance of being 

 native," though he makes the significant additional remark 

 that "none of the species are peculiar 2 ." The slug Testacella 

 makes a temporary cocoon, as do the land planarians and 

 some earthworms, in which no doubt it can survive drought. 

 But this will not sufficiently account for dispersal over 

 the sea ; and sea water is fatal to slugs and their eggs. 

 The eggs however are sometimes deposited in hollow 

 trunks of trees, where even the animals themselves occa- 

 sionally hide. The various methods of migration used 

 by other Molluscs may possibly be effective in the case of 

 slugs ; but at present there is here more than anywhere 

 a lamentable want of actual fact. 



Dispersal of Amphibia. 



The migrations of the larger aquatic animals are more 

 difficult ; and yet they have not unfrequently a wide 



1 Geographical Distribution of Slugs. P.Z.S. 1891, p. 214. 

 3 Cf. p. 152. 



B. Z. 10 



