MANUAL OF ZOOLOGY 



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

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

 ing. The shell of Arcella is of a transparent, tough material, 

 which is said to be chitinoid from the fact that it appears 

 to resemble a substance termed chitin^ of a horny consist- 



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

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

 and Wallfch.) 



ency, 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. 

 The bulk of the protoplasm is, as in Difflugia, enclosed 

 within the test, but a considerable portion of it may be 



