20 MANUAL OF ZOOLOGY sect. 



body within the shell (Stokes). An even commoner member 

 of the group is Arcella (Fig. 3, C). Arcella has a shell 

 much wider than that of Difflugia, convex on one side, flat on 

 the other. In the middle of the flat surface is a rounded 

 opening. The shell of Arcella is of a transparent, tough - 



Fig. 3.— A, Quadrula symmetrica; B, Hyalosphenia lata; C, Arcella vulgaris; 

 D, Difflugia pyriformis. (From Lang's Comparative Anatomy, after Schulze 

 and Wallich.) 



material, which is said to be chitinoid from the fact that it 

 appears to resemble a substance termed chitin, of a horny 

 consistency, very general in its occurrence in the integument 

 of animals. This chitinoid test exhibits a minute pattern 

 when examined under a high power of the microscope. 



