536 



ZOOLOGY 



SECT. 





they contract they move the mill in such a way that the three 

 teeth meet in the middle and complete the comminution of the 

 food begun by the jaws. The separation of the teeth is effected 

 partly by the elasticity of the mill, partly by delicate muscles in 

 the walls of the gizzard. The posterior division of the gizzard 



Fio. 444. Respiratory organs of Astacus fluviatilis. In A the gill-cover is removed and 

 the gills undisturbed ; in M tue podo branchiae are removed and the outer arthrobranchiae 

 turned down. a\, antennule ; 02, antenna ; ab\, first ; ab z , second abdominal segment ; 

 arb. 7 12, inner arthrobranchise ; arb'. 7 12 , outer arthrobranchise ; ep. 5, scaphognathite; 

 plb. 11 13, pleuro branchiae ; pdb. 7 13, podobranchs ; pi. 1, first pleopod ; 6 13, thoracic 

 appendages. (From Lang's Comparative Anatomy, after Huxley.) 



forms a strainer : its walls are thickened and produced into 

 numerous setae, which extend quite across the narrow lumen and 

 prevent the passage of any but finely divided particles into the 

 intestine. Thus the gizzard has no digestive function, but is 

 merely a masticating and straining apparatus. On each side of 

 the anterior division is found at certain seasons of the year a 

 plano-convex mass of calcareous matter, the gastrolith. 



