Chap. V. AND DENUDATION. 249 



lying the chalk (the residue of its dissolution 

 by rain-water) or from the chalk itself. 



Not only do worms aid indirectly in the 

 chemical disintegration of rocks, but there is 

 good reason to believe that they likewise act 

 in a direct and mechanical manner on the 

 smaller particles. All the species which 

 swallow earth are furnished with gizzards ; 

 and these are lined with so thick a chitinous 

 membrane, that Perrier speaks of it, # as " une 

 veritable armature." The gizzard is sur- 

 rounded by powerful transverse muscles, 

 which, according to Claparede, are about ten 

 times as thick as the longitudinal ones ; and 

 Perrier saw them contracting energetically. 

 Worms belonging to one genus, Digaster, 

 have two distinct but quite similar gizzards ; 

 and in another genus, Moniligaster, the 

 second gizzard consists of four pouches, one 

 succeeding the other, so that it may almost 

 be said to have five gizzards. f In the same 

 manner as gallinaceous and struthious birds 

 swallow stones to aid in the trituration of 



* ' Archives de Zoolog. exper.' torn. iii. 1874, p. 409. 

 f ' Nouvelles Archives du Museum,' torn. viii. 1872, p. 95, 

 131. 



