CHAPTER II 



THE TEST 



The covering of the animal in the f oraminifera is usually re- 

 ferred to as a test rather than a shell such as is secreted by 

 special organs in the Mollusca, etc. In the very simplest forms 

 of the Allogromiidae, the test is naked and without a distinct 

 test. In other primitive forms there is a more definite cell wall 

 but this may be broken through at any point. 



Chitinous Tests. The most primitive sort of definite test that 

 is developed is a chitinous one, often thin and transparent but 

 with a definite shape and aperture. Such tests occur in the 

 Allogromiidae but also occasionally in other groups such as 

 Pseudarcella, etc. 



Arenaceous Tests. In most of the earlier and more primitive 

 groups of the foraminifera there is an arenaceous or agglutin- 

 ated test made up of foreign material, sand grains, sponge 

 spicules, mica flakes, etc., loosely or firmly cemented together 

 over a thin chitinous inner layer representing the primitive 

 chitinous test of the still simpler groups. This outer material of 

 the arenaceous tests is of various sorts. In some of the most 

 primitive forms as in Astrorhiza for example (PI. 2, fig. 1) the 

 mud and sand of the ocean bottom in which the animal lives is 

 loosely cemented about channels leading to the central chamber. 

 Foreign bodies of all sorts, sponge spicules, other foraminifera, 

 etc., are included indiscriminately, and only the inner portion is 

 at all firmly cemented. The only purpose seems to be to form a 

 somewhat rigid protection about the softer protoplasmic body. 



Other less primitive forms show some power of selection in 

 that they take some general constituent of the bottom. Rhahdam- 

 mina in its various species usually uses sand grains while 

 Marsipella takes mostly sponge spicules. There does not seem 

 to be any high degree of selection, but a certain general constitu- 

 ent of the bottom is chosen and others discarded. 



