Foraminifera. Coloured scanning electron micrograph (SEM) of a selection of foraminiferans. Foraminiferans are marine single-celled protists that construct and inhabit shells (tests), which are composed of several chambers. They are one of the oldest phylogenetic groups (taxa) to continue existing. Foraminiferan fossils date back to the Cambrian period (about 540 million years ago) but may have existed for as long as 1.2 billion years. From a sample collected at 3000 meters from the North Altlantic Ocean. Magnification: x40 when printed 10 centimetres wide. Magnification: x250 when printed 10 centimetres wide.