A genus of marine protozoans in the class Acantharea. The kingdom Protozoa contains 18 phyla. One of the parvkingdoms (a hierarchical classification between kingdom and superphylum that is controversial and not officially recognized as such) is the Actinopoda (originally a class) containing two phyla, Heliozoa and Radiozoa. Within the Radiozoa is the class Acantharea, in which the genus Acanthometrida is placed. All members are unicellular planktonic marine protozoa with axopodia. Long slender pseudopodia are found in certain protozoans. Mitochondria are always present and contain flattened cristae. No cilia are present in the trophic phase. Members have a skeleton, made up of strontium sulfate (celestite) limited to 20 radially arranged rods that extend from the center, forming a characteristic pattern in which angles are quite exact, as in Acanthometra (see illustration). Since strontium sulfate is soluble in seawater, no acantharean fossils exist. The central endoplasm contains numerous nuclei and other organelles. The cytoplasm surrounding the spines contains a conical array of contractile microfilaments (myophrisks or myonemes that are Ca²⁺-activated) containing the contractile protein actin. The microfilaments expand or contract the gelatinous sheath surrounding the cell, a phenomenon apparently responsible for changes in level of flotation. Electron microscopic research with related genera shows that the microfilaments are arranged in two sets of bands: one set runs parallel to the axis of the spine, and the other runs crosswise, thus producing a cross-fibrillar network. Cysts and flagellated swarmers represent two stages of the life cycle, which are not completely known at present. The endoplasm often contains zooxanthellae (small dinoflagellates living in the protozoan's endoplasm).