CH. VII] SNAILS AND SLUGS 231 



gastropods are most numerous and occur among nearly 

 all classes of animals ; in fact the role assigned to snails 

 and slugs in the economy of the universe appears to be 

 the altruistic one of affording food to others. Rats, field- 

 mice, voles and hedgehogs prey upon various species of 

 Helix. Shrews attack the smaller species and young 

 individuals, while the water-shrew dives to hunt for 

 Limncea and Planorbis. Of birds the song- thrush is 

 perhaps the most persistent enemy of Helix, whose shell 

 the bird shatters by violent blows upon some favourite 

 sacrificial stone. Blackbirds, starlings, ducks, geese, coots 

 and landrails are all known to be devourers of slugs, 

 and many smaller birds that frequent reed-beds and 

 marshes feed freely upon the numerous small snails that 

 abound in such localities. Frogs and toads are notoriously 

 useful in clearing gardens of slugs and small snails, and are 

 also partial, in common with newts, to various aquatic or 

 semi-aquatic species. Trout and other fish are well known 

 to thrive and grow most rapidly in streams and ponds with 

 a dense gastropod population, both eggs and adults being 

 equally acceptable. Over 350 shells of one species of 

 Valvata have been taken from the stomach of a single eel. 

 Among invertebrate animals the Coleoptera provide 

 enemies both to fluviatile and terrestrial species : the great 

 water-beetle Dyticus marginalis is especially destructive 

 to Limnceidce, with an apparent preference for L. stag- 

 nalis; the Silphidce destroy smaller land species and are 

 stated by Cooke to break the shells by striking them 

 against their own prothorax. The glow-worm, Lampyris 

 noctiluca, and its allies, and the genera Cychrus, Carabus 



