This subgroup of protozoa includes the familiar shape-shifting amoebas, as well as heliozoa, radiozoa, and foraminifera (or forams for short).
Sarcodina are best known for their pseudopods (“false feet”) used for locomotion and feeding.
Amoeba radiosa; a Spirochaete bacterium is just visible at top right. Courtesy of James Evarts, Micscape Magazine.
Foraminifera bodies are made up of chalk extracted from sea water. Courtesy of Brian Darnton.
In amoebas, these pseudopods are generally lobe-like bulges that extend from the cell membrane. In heliozoa, radiozoa, and forams, the pseudopods more resemble needles or spikes sticking out from the cells.
Sarcodinas use their pseudopods to engulf or latch onto prey, which may include bacteria, algae or other protozoa. Many amoebas are active predators, oozing about on their false feet until they come into contact with a suitable meal. At that point, the pseudopod flows around and engulfs the hapless prey, until it is completely “swallowed.”
Heliozoa, radiozoa, and forams tend to be passive grazers and predators, relying on suitable food swimming or drifting past to come into contact with their pseudopods.
Several species in the sarcodina group, including some species of amoebas, cover themselves with protective shell-like coverings called tests. These tests are stippled with many small and large openings through which water can flow in and out and through which the pseudopods protrude.
The tests of radiozoa are made up of silica (the same substance in diatom cell walls) and can form very intricate, lacy designs that may be studded with long spines that increase buoyancy and ward off predators.
The tests of forams are made up of sand grains or organic compounds. These can become quite large, the biggest reaching a little over 2 inches in diameter, making them some of the largest single-celled organisms known.
When forams die, their tests sink and accumulate in large batches; the Great Pyramids of Egypt are built from sandstone composed largely of fossilized giant Nummulites, an ancient kind of foram. The famous White Cliffs of Dover are limestone cliffs formed from the skeletal remains of forams.