Categorized as a Eukaryotic Protist, the Protozoans differ from Algae in that they are all unicellular chemoheterotrophs.  The feeding methods of this group are very diverse, ranging from ingestion through phagocytosis or mouth-like structures, as well as with pseudopodia.  All have a trophozoite feeding and growing stage in their life cycles.

Phylum Euglenozoa
All organisms in this group are flagellated with crystalline rods.  They are only capable of asexual reproduction which is carried out through binary fission.

Class Euglenida
Organisms in this group possess flagella that arises anteriorly and are characterized by a pellicle composed of spiraling strips of proteins under the plasma membrane.  Their mitochondria contain disc-shaped cristae.
Euglena gracilis
This species, found in fresh water environments, display both autotrophic and heterotrophic behaviors.  In sunlight, their chloroplasts synthesize organic compounds through photosynthesis.  The chloroplasts within these cells are a result of a secondary endosymbiosis with green algae.  In the absence of light, E. gracilis lose their photosynthetic pigments and begin to feed exclusively on organic material, consuming food through phagocytosis, and using aerobic respiration to metabolize them.  When returning to the light, the photosynthetic pigments are restored and becomes photoautotrophic again.  They also contain eyespots that are used for phototaxis in which the cell moves in response to a light stimulus.  The eyespot is photo-sensitive and filters light at the base of the flagellum.

Class Kinetoplastida
This flagellated group is distinguished by the presence of a single mitochondrion containing a kinetoplast.  The kinetoplast is a unique structure made of multiple, circular DNA molecules and associated proteins.
Bodo saltans
B. saltans is a free-living flagellate found in both fresh water and marine environments where they prey on bacteria, engulfing them within a phagocytic vacuole.  This species has two flagella.  The first is short and projects from the anterior end.  The second, which is positioned at the posterior end, is much longer, extending beyond the length of the cell.  This posterior flagellum is primarily used to secure the organism to substrates in their aquatic environments.   Here they exhibit a unique twitching and jumping behavior which is characteristic of this species and is generated by flexing their posterior flagellum.  B. saltans is of particular medical importance to scientists because of its close relatedness to the highly pathogenic, parasitic group, the Trypanosomatids. By studying their mitochondrial genes, they hope to target the function of kinetoplasts for the medical treatment of trypanosomial and leishmanial diseases.

Phylum Amoebozoa
The Amoebozoans are characterized by the use of lobe-shaped pseudopods for locomotion and to engulf food.  Many are endocommensals or endoparasites, living in the digestive tracts of arthropods and vertebrates.
Iodamoeba butschlii
I. butschlii is an endocommensal in the large intestines, especially the cecal region, of primates and pigs.  While there is no pathology associated with infection, it is found in approximately 4-8% of the global population.  They exhibit a direct life cycle that includes trophozoite and cyst stages, both of which are voided in the stool; although, humans can only obtain this endocommensal by ingesting food or water contaminated with the cyst stage.  Once inside the intestines, I. butschlii excysts, transforming into the troph stage where they feed on fecal matter and reproduce by binary fission.  Differential diagnosis of this species is obtained by analysis of the nuclear morphology of the cyst stage only.  The cyst, which is ellipsoidal, contains only one nucleus which contains an eccentric endosome and crescent shaped periendosomal granules. 

Phylum Ciliophora
Organisms in Phylum Ciliophora contain numerous cilia and a layer of flattened vesicles which function to support their plasma membranes.  They also have two types of nuclei, a macronucleus and a micronucleus.

Class Ciliatea 

Paramecium aurelia
Found in many fresh water habitats, P. aurelia is hypertonic to their environment.  They solve this problem by means of specialized vacuoles that expel excess water from the cell that is constantly entering through osmosis.  Their cilia are used to sweep solid food into an oral groove which is then ingested via endocytosis, forming a food vacuole.  These vacuoles have a high acid content to aid in the digestive process.  Digested food can then pinch away from the food vacuole inside smaller vesicles.  The vesicles have a high surface area so that products of digestion may be easily absorbed by the rest of the cell and used in its metabolism.  P. aurelia also engage in a sexual behavior known as conjugation.  During this process, two Paramecia line up against each other, fuse at the oral groove, and exchange their nuclear material.  As a result, a genetic recombination even occurs in which each organism gives and receives equal amounts of DNA.

Class Opalinea

Opalina obtrigonoidea
O. obtrigonoidea are commensals which inhabit the rectum of frogs and occur in very high numbers, but do not exhibit any pathology.  They have a direct life cycle that displays unique temporal and host species specific aspects.  Their trophozoite stage exists throughout the majority of the year until spring approaches and the frog's hormones begin to change.  The change of hormones in the frog signals the trophs to encyst, becoming tomites.  The tomite, or cyst stage, only occur in spring, coinciding with the production of new tadpoles.  This strategy ensure that the tomites will find new hosts, continuing their life cycle.  After being ingested by a tadpole, sexual reproduction occurs in which flagellated female and male gametes fuse to form a zygote.  The zygote grows and attains maturity to a trophozoite the same time the frog metamorphoses into the adult form.