- Members of the phylum Sarcodina are distinguished by the presence of flowing extensions of cytoplasm called pseudopodia, which are used in feeding and, in some, for locomotion. The pseupseudopodia are given different names, depending on their shape and structure.
- Although organelles have remained relatively simple, many Sarcodina have evolved complex skeletons. The various classes of Sarcodina are distinguished by the nature of their skeletons and their pseudopodia.
- The marine, freshwater, and parasitic naked amebas have no special skeletal structures and possess large, com m only tubular lobopodia or small, straplike filopodia, which are used for both feeding and locomotion.
- Shelled am ebas, which are largely restricted to fresh water, are covered by a shell composed of secreted organic material or of foreign mineral material cemented together. A large aperture permits the protrusion of lobopodia or filopodia.
- Foraminiferans, which are largely benthic marine Sarcodina, possess a calcareous test that is usually multicham bered. A single large opening permits the protrusion of cytoplasm , which may cover the exterior of the test. Long, delicate, and often anastomosing reticulopodia extend from the protruded cytoplasm and are used in food trapping and locomotion.
- Heliozoans are spherical, radially arranged, floating, and benthic Sarcodina that are largely restricted to fresh water. Long, radiating, needle-like pseudopodia (axopods) are used in trapping food. The axopods arise from the interior (medulla) and extend through an outer ectoplasmic cortex, which is com m only vacuolated. The cortex often contains a siliceous skeleton of plates, tubes, and needles.
- Radiolarians are marine planktonic Sarcodina with spherical bodies and radiating axopods. An organic capsule wall separates a central cortex from extracapsular cytoplasm . Radiolarians have complex skeletons of silicon dioxide or strontium sulfate within the extracapsular cytoplasm , organized in the form of lattice spheres or radiating spines or both.