Manganese in Land and Fresh Water Mollnsca. 17 



investigate; the valuable observations of W. A. Gain 1 give 

 little positive information, since the questions of what snails 

 will eat in captivity and what they do eat in nature are ob- 

 viously distinct, except in so far as one may probably conclude 

 with propriety that what they will not eat under artificial 

 surroundings will at any rate not be a common food under wild 

 conditions. Examination of the stomach contents of freshly 

 caught specimens has occupied my attention for some time, 

 but here again it is hardly possible to distinguish between 

 what the snail meant to eat and the other stuff that he picked 

 up incidentally. Water snails may, for instance, contain 

 fragments of cellular plants with many diatoms, but we do 

 not know whether the snail picked up the diatoms in eating 

 phanerogamic tissue or vice versa or, indeed, whether what he 

 really wanted to eat leaves recognisable remains. To find 

 a snail habitually about some plant is not necessarily evidence 

 that he is there to eat that plant rather than the adherent 

 organisms ; the feeding tracks of Limncea and Planorbis are 

 admirably displayed in the brownish coating of the under side 

 of elderly water-lily leaves, and it seems likely that a particular 

 plant is regularly attended for its associated algae and the like 

 rather than for itself. In other cases, too, it is the dead or 

 partly decayed leaves which appear to attract particular 

 attention. 2 Our domesticated plants are relatively open to 

 attack by snails, and when one finds a couple of fat sowerbyi or 

 hortensis inside a particularly fine potato or prowling up a 

 lettuce, one has no doubt as to what they want to eat, and what, 

 in fact, they do eat. But most green plants seem to be, when 

 living, pretty satisfactorily protected, 3 and it is illegitimate 

 to conclude that all the snails one finds in a favourable nettle- 

 bed feed on nettles. It is also an open question whether each 

 species has any particular or favourite food. Arion ater will 

 eat pretty well anything — green plants, dead plants, fungi, 

 bread, earthworms, etc. — in nature ; Gain could not find any- 

 thing which Limax arbornm was prepared to enjoy. There is, 

 perhaps, every gradation between these extremes, but just 

 how each snail stands we do not know. 



Such being our state of ignorance, it is impossible with the 

 present data to solve the question. Certain points, however, 



1 Journ. Conch., Vol. VI. (1891), p. 349 ; The Naturalist 1889, p. 55 ; 

 see also H. W. Kew on the food of slugs ib. p. 103 and 1893, p. 145 ; R. F. 

 Scharff Set. Trans. Roy. Dublin Soc, Vol. IV. (1891) p. 513, and A. H. 

 Cooke in Molluscs and Brachiopods, 1895, p. 30. 



2 e.g. from H. pomatia, which, according to W. Jeffrey (J. E. Harting, 

 Rambles in Search of Shells, 1875, p. 72), is harmless to green plants in the 

 garden, which my own observations fully confirm. 



3 See J. W. Taylor Monograph Br. L. F. W. Mollusca, Vol. I. (1899), 

 pp. 286 ff. 



1917 Jan. 1. 



