Foraminifera

What are they?

The Foraminifera are single celled protists with shells. These ( the shells ) are divided into chambers, and more are added as it grows. As per the species, the shell material varies.

How do they live?

A fully grown one ranges from 100 micrometers to 20 centimetres. They may have one or more nuclei in it's cell. They have symbiotic relations with algae ( a surprising number of creatures do ) and protect them in their shells. Some eat small plankton, bacteria, organic molecules and a variety of other stuff.

The oldest fossil consists of simple tubes.

They have evolved into many forms associated with coral reefs. They were in large numbers when reefs flourished. The oldest fossil foraminifera, from the Cambrian, are simple agglutinated tubes. The tests have become widespread, but become extinct when lot of destruction of the coral took place ( or rather the organisms living there died ).

Their size makes them tough to see, but make them useful for study as there can be huge numbers found in tiny piece's of rocks. They are geologically short lived.