28 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



in which a covering or test is developed \ this test 

 may be chitinous (Gromia), or chitinous and calca- 

 reous, or, in rare cases, siliceous; and it may have either 



Fig. 2 c. Xiphacsutha, showing the siliceous skeleton. (After W. 



Tuomsou.) 



a single large orifice (Fig. 2 A), or the test may be per- 

 forated with a number of holes (Foramiiiifera), 



and may attain to a large size (Nummulites), and great 

 complexity of form. 



