Classification of Protozoa

Protozoa are defined as microscopic, acellular organisms that exist either singly or in colonies, characterized by a lack of tissues and organs and the presence of one or more nuclei. With approximately 50,000 known species within the Phylum Protozoa, these organisms exhibit a remarkable diversity and complexity despite their simple structure. Protozoa can be broadly categorized into two primary forms of life: free-living, found in various aquatic environments (including freshwater and seawater), and parasitic, which may reside externally (ectoparasites) or internally (endoparasites) within host organisms.

• Basic Structure and Organization:
  • Protozoans are unicellular, meaning their bodies consist of a single cell that performs all necessary functions for survival.
  • The body organization is at the protoplasmic grade, lacking complex tissues or organs.
  • Protozoa may have one or multiple nuclei, which can be either monomorphic (uniform in structure) or dimorphic (different in structure).
  • Their body can be either naked or encased in a pellicle, and some species are covered with shells or have an internal skeleton for structural support.
• Morphological Characteristics:
  • The shape of protozoan bodies varies widely, including forms that are spherical, oval, elongated, or flattened.
  • Body symmetry can be absent, bilateral, radial, or spherical.
  • The body form of many protozoa remains constant, while some change shape in response to environmental conditions or at different life stages.
  • The cellular protoplasm is differentiated into an outer layer called ectoplasm and an inner layer known as endoplasm.
• Vital Functions:
  • Protozoa carry out all essential life processes within their single-cell structure, demonstrating a subcellular division of labor.
  • Locomotion is achieved through various structures, such as pseudopodia (finger-like extensions), flagella (whip-like appendages), cilia (hair-like projections), or through a stationary existence.
  • Nutritional modes include:
    • Holozoic: ingesting solid organic material (animal-like).
    • Holophytic: utilizing photosynthesis (plant-like).
    • Saprozoic: absorbing dissolved nutrients from the environment.
    • Parasitic: deriving nourishment from a host.
• Physiological Processes:
  • Digestion occurs intracellularly within food vacuoles, where nutrients are processed.
  • Respiration is facilitated by diffusion across the cell’s surface, enabling gas exchange with the environment.
  • Excretion of waste products typically occurs through the general body surface, although some protozoa may excrete waste through temporary openings in the ectoplasm or permanent structures known as cytopyge.
  • In freshwater species, contractile vacuoles play a crucial role in osmoregulation, helping to maintain cellular water balance while also aiding in the removal of metabolic waste.
• Reproductive Strategies:
  • Reproduction in protozoa can be asexual or sexual:
    • Asexual reproduction occurs through methods such as binary fission, multiple fission, budding, or sporulation.
    • Sexual reproduction may involve conjugation (a form of gametic exchange) and the formation of gametes through syngamy.
  • Many protozoan life cycles are complex, involving alternating phases of asexual and sexual reproduction, referred to as alternation of generations.
• Survival Mechanisms:
  • Encystment is a survival strategy adopted by many protozoans, allowing them to endure adverse environmental conditions, such as food scarcity, extreme temperatures, or low moisture levels. This process also aids in dispersal to new habitats.
  • Unlike multicellular organisms, protozoa are not differentiated into somatoplasm (body material) and germplasm (reproductive material), which means they are not subject to the same patterns of natural death that affect more complex organisms.

Classification of Phylum Protozoa

The phylum Protozoa is diverse and complex, making classification challenging. Traditional classification systems, such as those by Hyman, Hickman, and Storer, recognize two subphyla based on locomotion, which are further divided into five classes.

Sub Phylum A: Plasmodroma

Subphylum A: Plasmodroma encompasses a diverse array of unicellular organisms that exhibit unique adaptations for movement, feeding, and reproduction. Members of this subphylum can possess different types of locomotory organelles, including flagella, pseudopodia, or may lack any form of locomotion entirely. The classification within this subphylum reflects a variety of life strategies, particularly in relation to their habitats and modes of nutrition.

• Locomotory Organelles:
  • The organisms within Plasmodroma utilize flagella or pseudopodia for movement. Some may exhibit a complete absence of locomotory structures, reflecting their specific ecological niches and evolutionary adaptations.
  • Flagella: These whip-like structures facilitate propulsion in a fluid medium. Organisms like Euglena are notable examples, utilizing one to many flagella for movement.
  • Pseudopodia: This flexible and extendable cellular projection enables organisms like Amoeba to move and capture food, exemplifying an adaptive strategy for survival in diverse environments.
  • Absence of Locomotory Organs: Certain members, particularly those that are parasitic, do not require locomotion due to their life cycle and habitat, as seen in the sporozoans.
• Nuclear Structure:
  • Organisms within this subphylum typically possess one kind of nucleus. This uniformity in nuclear structure may influence their reproductive strategies and life cycles.
• Classes Within Plasmodroma:
  • Class 1: Mastigophora:
    • Characterized by the presence of flagella, these organisms are capable of moving efficiently in aquatic environments.
    • Euglena serves as a prominent example, showcasing the dual ability to perform photosynthesis while also exhibiting heterotrophic characteristics when necessary.
  • Class 2: Sarcodina:
    • Members of this class utilize pseudopodia for both locomotion and food acquisition.
    • Amoeba exemplifies this class, where the extension and retraction of pseudopodia allow for movement and the engulfing of prey through a process known as phagocytosis.
  • Class 3: Sporozoa:
    • This class is defined by its lack of locomotory organs, as all members are parasitic.
    • Spore formation is a common reproductive strategy, allowing for survival and dissemination in host organisms.
    • Plasmodium, the causative agent of malaria, represents a significant member of this class, highlighting the impact of sporozoans on human health and ecology.

Sub Phylum B: Plasmodroma

Subphylum B: Plasmodroma is characterized by diverse unicellular organisms that exhibit distinct locomotion and nutritional adaptations. Members of this subphylum utilize specialized structures for movement and feeding, which play crucial roles in their survival and ecological interactions.

• Locomotory Organelles:
  • Organisms in Plasmodroma utilize either cilia or sucking tentacles for locomotion.
  • Cilia: These hair-like structures facilitate movement and aid in feeding by creating currents in the surrounding environment.
  • Sucking Tentacles: These structures are primarily present in the adult forms of certain organisms, allowing them to adhere to surfaces and capture prey efficiently.
• Nuclear Structure:
  • Members of Plasmodroma possess two types of nuclei, which contribute to their reproductive and metabolic processes.
  • The presence of both macronuclei and micronuclei is significant for the regulation of cellular functions and genetic variation during reproduction.
• Classes Within Plasmodroma:
  • Class 4: Ciliate:
    • Characterized by the use of cilia for movement, members of this class are well-adapted to aquatic environments.
    • Paramecium serves as a key example, demonstrating rapid movement through coordinated ciliary action. This organism showcases the effective use of cilia not only for propulsion but also for feeding through the creation of feeding currents that direct food particles toward the cytostome.
  • Class 5: Suctoria:
    • This class includes organisms that move by cilia in their juvenile stages and develop tentacles as adults, which are utilized for feeding.
    • An example is Podophyra, which exemplifies this life cycle transition. The young forms possess cilia, allowing them to swim freely, while the adult forms adopt a sessile lifestyle, using sucking tentacles to extract nutrients from their environment.

Subphylum I: Sarcomastigophora

Subphylum I: Sarcomastigophora encompasses a diverse group of unicellular organisms characterized by their modes of locomotion and reproductive strategies. Members of this subphylum can utilize either pseudopodia or flagella for movement, which plays a vital role in their feeding and interaction with the environment.

• Locomotor Organelles:
  • Pseudopodia: These temporary, foot-like extensions of the cell allow for movement and the capturing of food. Organisms that employ pseudopodia, such as Amoeba, demonstrate a unique method of locomotion known as amoeboid movement, where the cell changes shape to facilitate movement and prey capture.
  • Flagella: Some members utilize one or more whip-like structures, known as flagella, for propulsion through their aquatic habitats. This form of locomotion is efficient for organisms such as Euglena, which can swim rapidly and change direction with agility.
• Nuclear Structure:
  • Organisms within Sarcomastigophora possess a single type of nucleus, referred to as monomorphic. This uniformity indicates a simplified genetic structure, which may influence their reproductive mechanisms and overall cellular functions.
• Reproductive Strategies:
  • Unlike some other protozoans, members of this subphylum do not engage in spore formation, which is often associated with parasitic life cycles. Instead, reproduction primarily occurs through syngamy, a process involving the fusion of two gametes. This sexual reproduction mechanism facilitates genetic diversity and adaptation in changing environments.

Superclass A: Mastigophora

Superclass A: Mastigophora, commonly referred to as flagellates, is a diverse group of unicellular organisms characterized by their use of flagella for locomotion. This superclass includes both free-living species and those that are parasitic, reflecting a wide range of ecological adaptations and nutritional strategies.

• Locomotory Organelle:
  • In adult forms, flagella serve as the primary locomotory organelles. These whip-like structures enable effective movement through various aquatic environments, allowing flagellates to navigate and access nutrients efficiently.
• Body Structure:
  • The body of Mastigophora is typically covered by a pellicle, which provides structural support and protection. This flexible layer allows for some shape alteration while facilitating the movement of the organism.
• Reproductive Strategy:
  • Reproduction in Mastigophora predominantly occurs through longitudinal binary fission. This process involves the division of the organism along its length, resulting in two genetically identical offspring.
• Nutritional Modes:
  • Members of this superclass exhibit diverse nutritional strategies, including autotrophic, heterotrophic, or mixotrophic modes of nutrition. Autotrophic species, such as those in the Class Phytomastigophorea, utilize photosynthesis to obtain energy, while heterotrophic species consume organic material for sustenance.
• Classes Within Mastigophora:
  • Class 1: Phytomastigophorea:
    • These organisms contain chlorophyll-bearing chromatophores, enabling them to engage in holophytic nutrition primarily through phototrophy.
    • Their reserve food is typically stored as starch or paramylon, providing energy when photosynthesis is not possible.
    • Most members possess one or two flagella, and their nucleus is generally vesicular in structure.
    • Orders within this class include:
      • Chrysomonadina: Examples include Chromulina and Ochromonas.
      • Coccolithophorida: Examples include Coccolithus.
      • Heterochloride: Examples include Heterochloris.
      • Cryptomonadida: Examples include Chilomonas.
      • Dinoflagellida: Examples include Noctiluca and Ceratium.
      • Euglenida: Notable examples are Euglena and Phacus.
      • Volvocida (Phytomonadida): Examples include Volvox and Chlamydomonas.
      • Chloromonadida: Examples include Vacularia.
  • Class 2: Zoomastigophorea:
    • Members of this class lack chlorophyll and chromatophores, making them predominantly heterotrophic.
    • They are mostly parasitic, deriving nutrients from host organisms.
    • Their reserve food is stored as glycogen, a polysaccharide that serves as an energy source.
    • Flagella in this class can range from one to many, and many species possess an undulating membrane that aids in movement.
    • Orders within this class include:
      • Choanoflagellida: Example is Proterospongia.
      • Rhizomastigida: Examples include Mastigoamoeba.
      • Hypermastigida: Examples include Trichonympha.
      • Diplomonadida: Notable examples include Giardia.
      • Kinetoplastida: Suborder examples include Trypanosoma and Leishmania.
      • Bicosoecida: Examples include Salpingoeca.
      • Retortamonadida: Example is Chilomonas.
      • Oxymonadida: Examples include Oxymonas.
      • Trichomonadida: Example is Trichomonas.

Superclass B: Opalinata

Superclass B: Opalinata represents a distinct group of unicellular organisms characterized by their unique structural features and parasitic lifestyle. These organisms primarily inhabit the intestines of amphibians, particularly frogs and toads, where they play specific ecological roles within their host environments.

• Locomotion and Structure:
  • Opalinates possess numerous cilia-like organelles arranged in oblique rows that cover the entire body surface. This arrangement enables effective movement through the viscous environment of their hosts, allowing them to navigate and adhere to intestinal linings.
• Nuclear Composition:
  • These organisms are characterized by the presence of two or more monomorphic nuclei. This uniformity in nuclear structure facilitates the organism’s cellular functions and contributes to its overall biological efficiency.
• Nutrition and Feeding Mechanisms:
  • Notably, opalinates lack a cytostome, the mouth-like structure typically found in many other protozoans. This absence indicates that they may absorb nutrients directly through their body surface or rely on the host’s digestive processes to obtain necessary sustenance.
• Reproductive Strategy:
  • Reproduction in Opalinata primarily occurs through binary fission, which is interkinetal in nature. This process involves the organism undergoing division while maintaining a specific arrangement of nuclei and cytoplasmic structures.
  • Additionally, sexual reproduction occurs via syngamy, where flagellated anisogametes fuse. This reproductive strategy enhances genetic diversity and adaptability within parasitic environments.
• Ecological Role and Habitat:
  • All members of Superclass B are parasitic, predominantly residing within the gastrointestinal tracts of frogs and toads. This parasitism can impact host health and digestion, potentially influencing the broader ecosystem dynamics in which these amphibians exist.
  • Examples of opalinates include Opalina, Protoopalina, Zelleriella, Protozelleriella, and Cepedea, which collectively exemplify the structural and functional diversity within this superclass.

Superclass C: Sarcodina

Superclass C: Sarcodina encompasses a diverse group of unicellular organisms that exhibit unique adaptations primarily characterized by their mode of locomotion and nutritional strategies. These organisms play significant roles in various ecological systems and can be found in both aquatic and terrestrial environments.

• Locomotion and Structural Features:
  • Sarcodines primarily utilize pseudopodia for movement. These extensions of the cell body allow for creeping locomotion, enabling the organism to navigate its surroundings effectively.
  • The predominant form within this superclass is amoeboid, characterized by the flexible and adaptive nature of their body structure. Some sarcodines possess a hard shell, which provides protection and support.
• Reproductive Strategies:
  • Sarcodines typically do not form spores, a feature that distinguishes them from other protozoan groups. Instead, they commonly produce gametes and flagellated young. This life cycle can involve both sexual and asexual reproduction, contributing to their adaptability in varying environments.
• Nutritional Modes:
  • Nutrition among sarcodines is generally holozoic or saprozoic. Holozoic nutrition involves the ingestion of solid food particles through phagocytosis, while saprozoic nutrition pertains to the absorption of dissolved organic matter from the environment.
• Classifications within Sarcodina:
  • Class 1: Rhizopodea
    • This class is marked by the presence of pseudopodia, either lobopodian or filopodia, but never axopodia. The organisms within this class are typically creeping forms.
    • Subclass a: Lobosia
      • Organisms possess lobopodian pseudopodia.
      • Order 1: Amoebida: Examples include Amoeba, Entamoeba, and Pelomyxa.
      • Order 2: Arcellinida: Examples include Arcella, Diffugia, and Euglypha.
    • Subclass b: Filosia
      • Characterized by tapering and branching filopodia.
      • Examples include Gromia, Allogromia, and Penardia (naked).
    • Subclass c: Granuloreticulosia
      • Features finely granular reticulose rhizopodia (reticulopodia).
      • Order 1: Foraminiferida: Examples include Globigerina and Elphidium.
    • Subclass d: Mycetozoia
      • This subclass showcases complex life cycles, wherein the amoeboid trophic stage can develop into either a multicellular aggregation or a true multinucleate plasmodium.
      • Sporangia are commonly formed, liberating spores.
      • Nutrition is phagocytic, with Plasmodiophora serving as a notable example.
• Class 2: Actinopodea
  • Members of this class are characterized by axopodia, which have axial filaments radiating from a spherical body. They are mostly sessile or floating forms.
  • Gametes are typically flagellated, and reproduction occurs through both sexual and asexual methods.
    • Subclass a: Radiolaria
      • Features a central capsule perforated by numerous pores, housing spicules or a siliceous skeleton.
      • Filopodia or axopodia are present, with marine examples including Thalassicola, Collozoum, and Lithocircus.
    • Subclass b: Acantharia
      • Possesses an imperforate, non-chitinoid central capsule without pores and an anisotropic skeleton of strontium sulfate.
      • Examples include Acanthometra.
    • Subclass c: Heliozoia
      • Characterized by a rounded body with radiating axopodia and a lack of a central capsule.
      • They may exhibit multiple nuclei and are mostly found in freshwater. Examples include Actinophrys, Actinosphaerium, and Clathrulina.
    • Subclass d: Proteomyxidia
      • Comprising largely marine and freshwater parasites of algae and higher plants, these organisms exhibit filopodia and reticulopodia.
      • Examples include Vampyrella and Pseudospora.
• Class 3: Piroplasmea
  • This class consists of small, round-shaped or amoeboid parasites primarily found within vertebrate red blood cells. A well-known example is Babesia, which illustrates the parasitic nature of some sarcodines.

Subphylum II: Sporozoa

Subphylum II: Sporozoa comprises a diverse group of protozoan parasites primarily known for their endoparasitic lifestyle. These organisms are characterized by their lack of locomotory organelles, reliance on spore formation, and complex reproductive strategies. Sporozoa are significant in both medical and ecological contexts due to their roles as parasites in various hosts.

• General Characteristics:
  • Sporozoa are exclusively endoparasitic, meaning they inhabit the internal environments of their hosts.
  • They do not possess locomotory organelles; hence, they do not move independently.
  • Cilia or flagella may be present in gametes, allowing for movement during the reproductive phase.
  • The reproductive process typically involves syngamy, leading to the formation of numerous spores.
  • Spores are usually simple structures that may contain one to many sporozoites, which represent the infective phase of the life cycle.
  • Nuclei within Sporozoa are of a single type, indicating a lack of nuclear diversity.
• Classifications within Sporozoa: Sporozoa is further divided into distinct classes, each characterized by unique reproductive and developmental features.
  • Class 1: Telosporea
    • In this class, pseudopodia are generally absent, and locomotion occurs through gliding or body flexion.
    • Spores are produced, and in some species, flagellated microgametes are present.
    • Spores lack polar capsules and filaments, existing in either naked or encysted forms.
    • Reproduction occurs through both sexual and asexual methods, which enhances the adaptability of these organisms.
    • Subclass a: Gregarinia
      • Mature trophozoites are large and typically exist extracellularly.
      • Reproduction is entirely sexual through a process called sporogony, resulting in spores that contain eight sporozoites.
      • These organisms are primarily parasites of the digestive tract and body cavity of invertebrates, with examples including Gregarina, Monocystis, and Nematocystis.
    • Subclass b: Coccidia
      • Mature trophozoites in this subclass are smaller and generally intracellular.
      • Each oocyst produces numerous sporozoites, which facilitates their proliferation within host organisms.
      • They are parasites of the digestive tract or blood of vertebrates, with gametocytes displaying dimorphic characteristics.
      • Sporozoites multiply through schizogony within tissue cells.
      • Order 1: Eucoccida
        • In this order, schizogony occurs, and both sexual and asexual phases are present, affecting epithelial and blood cells of various hosts.
        • Suborder 1: Eimeriina
          • In this suborder, macrogametes and microgametocytes develop independently, and there is no occurrence of syzygy.
          • The macrogametocyte produces many microgametes, leading to a zygote that remains non-motile.
          • Oocysts do not increase in size during sporogony, with sporozoites encased in sporocysts. An example is Eimeria.
        • Suborder 2: Haemosporina
          • Similarly, macrogametes and microgametocytes develop independently without syzygy.
          • Microgametocytes produce fewer microgametes, and the resulting zygote is often motile.
          • Oocysts increase in size during sporogony, with naked sporozoites. Schizogony occurs in vertebrates, while sporogony takes place in an invertebrate host. An example is Plasmodium, known for its role in malaria.
  • Class 2: Toxoplasmea
    • This class is characterized by the absence of spores and the lack of flagella or pseudopodia at any stage of development.
    • Reproduction occurs solely through asexual methods, specifically binary fission, which results in the formation of cysts containing multiple naked sporozoites.
    • Notable examples include Sarcocystis and Toxoplasma.
  • Class 3: Haplosporea
    • In this class, spores are present, and while pseudopodia may exist, flagella are absent.
    • Reproduction occurs exclusively through asexual methods, with schizogony being the primary reproductive strategy.
    • Examples include Caelosporidium and Ichthyosporidium.

Subphylum III: Cnidospora

Subphylum III: Cnidospora represents a specialized group of protozoan parasites characterized by their unique spore structures and developmental processes. These organisms are predominantly endoparasitic, meaning they thrive within their hosts, and exhibit fascinating adaptations that allow them to infect a variety of organisms, including fish and arthropods.

• General Characteristics:
  • Cnidospora are distinct for their spores, which are multicellular and contain one or more polar filaments. These filaments are coiled threads capable of being shot out, facilitating the infection of host cells.
  • Each spore also contains one or more sporoplasms or sarcoplasms, analogous to sporozoites, which are the infective forms that enter host tissues.
  • All members of this subphylum are parasitic, establishing themselves within the tissues of various hosts.
  • The zygote develops into one or more trophozoites without undergoing sporogony, which is a notable deviation from the reproductive strategies seen in other protozoan subphyla.
• Classifications within Cnidospora: This subphylum is further divided into two main classes, each exhibiting unique morphological and ecological traits.
  • Class 1: Myxosporidea
    • Spores in this class are of multicellular origin and are relatively large compared to those in other classes.
    • Each spore typically features one or more sporoplasms, which are protected by two or three valves.
    • Myxosporidia primarily parasitize fish, and their life cycles often involve complex interactions with aquatic hosts.
    • Examples include Myxobolus, Myxidium, and Ceratomyxa, all of which exemplify the diverse adaptations of these parasites to their aquatic environments.
  • Class 2: Microsporidea
    • In contrast, Microsporidea spores are of unicellular origin and are notably smaller.
    • These spores possess a single long tubular polar filament, through which sporoplasms emerge; they typically have only one valve.
    • Microsporidia are cytozoic, meaning they are intracellular parasites that infect arthropods and vertebrates, displaying a wide range of host specificity.
    • A prominent example of this class is Nosema, which has significant implications in the health of various host organisms, particularly in agriculture and aquaculture.

Subphylum IV: Ciliophora

Subphylum IV: Ciliophora encompasses a diverse group of protists characterized by their unique ciliary structures, complex life cycles, and dual-nucleus system. These organisms play significant roles in aquatic ecosystems and exhibit a variety of nutritional strategies.

• General Characteristics:
  • Ciliophora organisms possess simple ciliary organelles that facilitate locomotion and feeding.
  • They exhibit a distinct infraciliature, which is subpeculiar and consists of basal granules located beneath the cell surface, interconnected by longitudinal fibrils.
  • These protists contain two types of nuclei: a macronucleus responsible for trophic functions and a micronucleus involved in reproduction.
  • The reproduction process typically involves binary fission, which is perkinetal, meaning that it occurs along the longitudinal axis of the cell.
  • Sexual reproduction occurs through conjugation, where nuclei fuse; however, autogamy and cytogamy are also mechanisms for genetic exchange.
  • Notably, there are never any free gametes produced.
  • Nutrition is classified as mixotrophic or heterotrophic, with most ciliates having a cytostome, or cell mouth, for food intake.
• Class 1: Ciliata
  • Members of this class possess cilia or complex ciliary structures, which serve as both locomotory and feeding organelles.
  • A permanent anal aperture, known as cytopyge, is present, facilitating the expulsion of waste.
  • The infraciliary system is a key feature, enhancing the cell’s structural integrity and function.
  • Ciliata exhibit transverse fission for asexual reproduction.
  • Despite their diverse forms, sexual reproduction never results in the formation of free gametes.
  • Contractile vacuoles are typically present, allowing for osmoregulation, even in marine and parasitic forms.
  • Subclass 1: Holotricha
    • Characterized by simple and uniform body cilia, this subclass lacks buccal cilia.
    • Order 1: Gymnostomatida
      • Examples include Coleps, Dileptus, Didinium, Prorodon, and Nassula.
    • Order 2: Trichostomatida
      • Notable examples are Colpoda and Balantidium.
    • Order 3: Chonotrichida
      • Examples include Spirochona, Lobochona, and Chilodochona.
    • Order 4: Apostomatida
      • A representative example is Hyalophysa.
    • Order 5: Astomatida
      • This order includes Anoplophyra, Maupasella, and Hoplitophyra.
    • Order 6: Hymenostomatida
      • Common examples are Colpidium, Tetrahymena, and Paramecium.
    • Order 7: Thigmotrichida
      • An example from this order is Thigmophyra and Boveria.
  • Subclass 2: Peritricha
    • Adults in this subclass typically lack body cilia, with the apical end exhibiting buccal cilia.
    • Order 1: Peritrichida
      • Examples include Vorticella, Carchesium, and Trichodina.
  • Subclass 3: Suctoria
    • This group is characterized by a sessile and stalked body form.
    • Young individuals possess cilia, while adults develop suctorial tentacles for feeding.
    • Order 1: Suctorida
      • Examples include Acineta, Ephelota, and Podophyra.
  • Subclass 4: Spirotrichia
    • In this subclass, body cilia are reduced, but buccal cilia are well-defined.
    • Order 1: Heterotrichida
      • Notable examples are Stentor, Bursaria, and Spirostomum.
    • Order 2: Oligotrichida
      • Examples include Halteria and Strombidium.
    • Order 3: Tintinnida
      • Examples consist of Codonella and Favella.
    • Order 4: Entodinomorphida
      • Examples include Entodinium and Cycloposthium.
    • Order 5: Odontostomatida
      • An example is Saprodinium.
    • Order 6: Hypotrichida
      • This order includes Euplotes, Stylonychia, Urostyla, and Oxytricha.