Protozoa are ancestral to the other eukaryote kingdoms rather than a complete evolutionary group in themselves. Traditionally most single-celled organisms that feed by ingestion were placed here, but many of the best-known belong with the Chromista, leaving a somewhat heterogeneous collection of groups that do not fit anywhere else.
Greek heteros, different, lobos, lobe
These are mainly small protozoa that transform between flagellates in rapid movement, and amoebae in feeding. However some only have one form or the other.
The amoebae are eruptive, moving through rapid bursts along the front margin. This separates them from other groups which move by continuous streaming.
Euglenozoa are a large group of flagellates that share a common cell structure with 1-4 flagella containing internal rods, which arise from a depression and are often associated with a mouth. However, these features are not apparent in many smaller types, which are mostly bacterivores or endosymbionts.
Euglenoids include some more conspicuous kinds, most common in nutrient-rich water. Their flagella are set in an anterior reservoir, and their outer membrane is supported by longitudinal strips, forming a pellicle that helps them maintain or change their shape. Different types feed by ingestion, absorption, or photosynthesis.
Phagotrophic euglenoids usually have feeding rods to help them ingest food such as bacteria, other euglenoids and algae, or dead material. They often have one leading and one trailing flagellum, with different types using one or the other to glide over surfaces.
Greek pēra, pouch, nēma, thread
Peranema have a stiff leading flagellum, held forwards and beating only near the tip. The other is fused to the cell, which is plastic and cylindrical with a rounded posterior.
Greek anisos, unequal, nēma, thread
Anisonema are oval with a deep reservoir behind the front. The trailing flagellum runs back through a furrow and well past the cell, so that it can quickly pull back and change direction.
Photosynthetic euglenoids are mostly bright green, with small chloroplasts originally taken from green algae and a red eyespot near the flagella. Most belong to the Euglenida, where only one flagellum extends out of the reservoir. This undergoes a looping motion, pulling them along a slightly helical path.
Greek lepos, rind, kigklis, lattice
Lepocinclis are ovoid to long with small plastids and a sharp posterior spine. The pellicle may be capable of bending but not deforming and often has prominent helical grooves.
Greek phakos, lentil or lens
Phacus are mostly broad and leaf-shaped with a tail spine. Such cells are either flattened or divided into thin lobes, often somewhat twisted around the main axis.
Greek eus, good, glēnē, eyeball
Euglena have a deformable pellicle allowing metaboly, a slow creeping movement by worm-like contractions. They are mainly distinguished by plastid or molecular features.
These vary in size and form, and some have blunt or acute ends without the tapering spine typical of similar genera. They also include some that become red in certain conditions.
Greek kolax, flatterer, -ion, dimn.
Colacium live attached to small crustaceans or other objects, most forming branching colonies with relatively thick stalks. The cells are oval with an eyespot near the base.
Greek trachēlos, neck, monas, unit
Trachelomonas are enclosed by a protective lorica, which is rounded or spiny and usually brownish. This has a small opening for the flagellum, often with a short but distinct collar.
Amoebozoa generally move by internal flow of the cell body, which happens more or less continuously, rather than thinner pseudopodia or eruptions. They have no constant shape, but most have a fairly characteristic type of form during directed movement. Many of the largest protozoa, like slime molds, belong here.
Discosea are flattened amoebae that move as a whole, with a clear front margin that acts as a single pseudopodium. Some also have more or less pointed sub-pseudopodia, projections that are not directly involved in movement, or a protective coating.
Greek thēkē, case, amoibē, change
Thecamoeba are longer than wide and have a thickened coat. This is reflected by dorsal folds, either in parallel ridges running along the back or as irregular wrinkles in larger types.
Tubulinea are typically amoebae that move using blunt projections, called lobopodia. Smaller kinds are usually cylindrical but may spread out during feeding. Larger ones have multiple lobopodia, and often form protective shells from various secretions or collected debris.
Greek polys, many, chaos, unformed mass
Polychaos are mainly short and palmate with a number of lobopodia of similar size. These can flow together as the cell moves, instead of all but one eventually retracting.
After Harald Netzel, published c. 1970s
Netzelia have granular shells of spherical to ovoid shape. There is a round opening, and often a thickened rim, with a margin of lobes or inward teeth. Some also have several spines.
These often collect mineral particles for their shell, but can also produce fine silicate granules or organic material.
Latin arca, box, -ella, dimn.
Arcella have round cap-shaped shells opening in a central circle. They are made entirely from secreted material, starting out clear and becoming brown and dark as they age.
Greek kentron, tip, pyxis, box
Centropyxis make wedge-shaped shells with both secreted and collected material. The opening is set near the front and they often have several spines along the back margin.