Introduction
The second subphylum of Sarcomastigophora: protozoa that move and feed with pseudopodia. Medically, one species matters most: Entamoeba histolytica, the cause of amoebic dysentery and amoebic liver abscess. Knowing the harmless look-alikes (E. dispar, E. coli) and the dangerous free-living amoebae completes the topic.
Subphylum Sarcodina: general features
- Move and feed with pseudopodia ("false feet"): temporary outflows of cytoplasm, driven by actin and myosin. The cytoplasm switches between a stiff gel (ectoplasm) and a fluid sol (endoplasm), which flows forward into the pseudopodium (amoeboid movement).
- Types of pseudopodia: lobopodia (broad, blunt: Amoeba, Entamoeba), filopodia (thin), reticulopodia (branching networks: Foraminifera) and axopodia (stiffened by microtubules: Radiolaria, Heliozoa).
- No pellicle, so no permanent shape (except the shelled forms). The cytoplasm has a clear outer ectoplasm and a granular inner endoplasm with the nucleus, food vacuoles and, in fresh-water species, a contractile vacuole.
- Feed by phagocytosis (engulfing bacteria, algae, host cells) and pinocytosis.
- Reproduce asexually by binary fission; many form cysts to survive dry or hostile conditions and to spread.
- Most are free-living in water and soil; shelled marine forms (Foraminifera, Radiolaria) built limestone and chalk deposits over geological time.
- Medically important: Entamoeba (intestinal parasites) and the free-living amoebae Naegleria, Acanthamoeba and Balamuthia, which cause rare but severe infections. (In modern taxonomy Entamoeba and Acanthamoeba belong to the Amoebozoa, while Naegleria is an excavate: "amoeba" describes a way of life, not one lineage.)