Any of various chiefly marine protozoans of the order Foraminiferida or Foraminifera, having a body enclosed by a shell called a test and making up an important constituent of plankton. Perforations in a foraminiferan's test allow the protrusion of numerous long extensions (pseudopods),
which form a net used to trap food. The tests of foraminiferans grow throughout the organism's life, and can exceed 5 cm (2 inches) in diameter. The tests of dead organisms form ooze found on the ocean floor. Extinct foraminiferans are important index fossils.