Chap. II. DEPTH OP THEIE BUEEOWS. 109 



ordinary vegetable mould, though he admits 

 that they might be nourished to some extent 

 by leaf-mould.* But we have seen that 

 worms eagerly devour raw meat, fat, and 

 dead worms ; and ordinary mould can hardly 

 fail to contain many ova, larvae, and small 

 living or dead creatures, spores of crypto- 

 gamio plants, and micrococci, such as those 

 which give rise to saltpetre. These various 

 organisms, together with some cellulose from 

 any leaves and roots not utterly decayed, 

 might well account for such large quantities 

 of mould being swallowed by worms. It 

 may be worth while here to recall the fact 

 that certain species of Utricularia, which grow 

 in damp places in the tropics, possess bladders 

 beautifully constructed for catching minute 

 subterranean animals ; and these traps would 

 not have been developed unless many small 

 animals inhabited such soil. 



The depth to which worms penetrate, and 

 the construction of their burrows. — Although 

 worms usually live near the surface, yet they 

 burrow to a considerable depth during long- 



* 'Zeitschrift ftir wissenschaft. Zoolog.' B. xxviii. 1877 

 p. 364 



