314 CONCLUSION. CHAP. VIT. 



Hensen* placed two worms in a vessel 

 18 inches in diameter, which was filled with 

 sand, on which fallen leaves were strewed ; 

 and these were soon dragged into their bur- 

 rows to a depth of 3 inches. After about 

 6 weeks an almost uniform layer of sand, a 

 centimeter (*4 inch) in thickness, was con- 

 verted into humus by having passed through 

 the alimentary canals of these two worms. 

 It is believed by some persons that worm- 

 burrows, which often penetrate the ground 

 almost perpendicularly to a depth of 5 or 6 

 feet, materially aid in its drainage ; notwith- 

 standing that the viscid castings piled over 

 the mouths of the burrows prevent or check 

 the rain-water directly entering them. They 

 allow the air to penetrate deeply into the 

 ground. They also greatly facilitate the 

 downward passage of roots of moderate size ; 

 and these will be nourished by the humus 

 with which the burrows are lined. Many 

 seeds owe their germination to having been 

 covered by castings; and others buried to 

 a considerable depth beneath accumulated 

 castings lie dormant, until at some future 



* ' Zeitschrift fur wissenscbaft. Zoolog.' B. xxviii. 1877, p. 36(X 



