Although the tests of foraminifers are used extensively by paleontologists as paleoecologic indicators, very little is known of the ecology of modern foraminifers, and virtually nothing is known of the function of the test.

Investigation of test-protoplasm relations have been hampered by lack of suitable techniques for making meaningful observations. Now the scanning electron microscope provides a means of examination of the test on a scale which permits understanding of spatial relations of the test and the living organism. Coupled with experiments and observations on specimens in laboratory culture, the information yielded by studies of test structure and of fixed, frozen and dried protoplasm suggests a general theory of test function.

1. The most primitive tests are constructed of arenaceous material to provide weight to counteract bouyancy of the protoplasm. The simplest arenaceous tests seem to serve only this function.

2. The test upon elaboration into a long tube or series of chambers separated by narrow openings, serves as an effective barrier to retard the effects of unfavorable changes in environmental chemistry.

3. Further specialization may adapt the test for growth under special physical conditions, such as a certain substrate, or for particular symbiotic conditions, such as the greenhouse function of the test of Elphidium.

The role of the test as a protective device against predation is not understood, but may account for some specialized forms.

The great variety of test form in the foraminifers suggests that many taxa are particularly well adapted for specialized ecologic niches. The fact that the foraminifers are among the hardiest of marine protozoa and almost unique in their ability to withstand changes in the environment indicates that they have developed a highly efficient means of controlling their immediate environment without encystment or metamorphosis; that means is the test.

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