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. expeV torn. iii. 1874, p. 409. 

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

 131. 



