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In this lesson, we’ll explore the world of animal-like protists. We’ll first discuss what it means to be a protist, then review characteristics that define a protist, and finally, look at a few examples of these widely-ranging organisms.

What Is a Protist?

Alright, so what is a protist? Well, protists are a kingdom of single-celled eukaryotic organisms (meaning they have membrane-bound organelles like lysosomes and a nucleus) that can be either solitary living or colonial and can’t be classified as belonging to the kingdoms of plants, animals, bacteria, or fungi. Now, I know it might seem funny to classify them by what they are not (not animals, not plants, not fungus, or bacteria), and many biologists would agree with you. But protists are a bit problematic when it comes to taxonomic classifications.The problem with protist classification can be traced back to their genes. Not blue jeans, but their genetic makeup.

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Geneticists have, in more recent years, found that protists are polyphyletic, meaning that the group, as a whole, does not share a common ancestor. In other words, protists do not possess the defining characteristics that define an organism as belonging to a certain taxonomic group, such as birds having feathers or sharks having cartilaginous skeletons. Their genes show that they are more closely related genetically to organisms in other kingdoms than they are to one another.

Talk about a classification nightmare! To manage this, biologists generally discuss protists based on what they eat, how they move (having cilia, a flagellum, or being amoeboid), or where they live (marine, freshwater, soil).

Eating Habits

In other words, plant-like protists share the plant-like quality of being autotrophic, or producing their own food through photosynthesis. In contrast, animal-like protists (known as protozoans, or ‘first animals’) share heterotrophic behaviors, meaning they acquire their nutrients by exhibiting a very animal-like hunting behavior and consume bacteria or other smaller protists. Therefore, protozoans are protists, but not all protists are protozoan.Now, what you’ll find in exploring a few examples of these organisms, is that the way they feed is pretty much all that some of these protists have in common. What’s more is that their animal-like status garnered them the special name, protozoan, meaning ‘first animal,’ which isn’t used to mean that they are the first representation of animals or that animals stemmed from them but that they are the principle hunters of the microbial world.

Heterotrophic Protists

Heterotrophic protists run the gamut in size, shape, and appearance. However, what unites them is not only their heterotrophic status but also that they are freely motile. Because they hunt, they must be able to move actively in their environment and to do this, they have specialized motility structures. Because of this, biologists will often discuss heterotrophic protists in terms of their mode of movement.

Modes of Movement

Paramecium are a variety of ciliated protozoan, meaning their bodies are covered in hair-like, cilia projections that they beat against their watery environment to propel themselves towards their prey. They can range in size from the small 50 micrometers (0.002 inches) to about 330 micrometers (or 0.

013 inches) whereas other heterotrophic ciliates, such as marine and freshwater dwelling Spirostomum, can grow as large as 4 millimeters.Flagellated protists are those which have at least one (and sometimes multiple) tail-like projections, called flagellum which they whip to propel themselves forward. Some flagellates, like Parabasala, are symbiotes with host organisms, living in their guts and aiding in digestion, while other are parasitic. Leishmania and Giardia are common examples of parasitic flagellates, and they can cause skin sores, organ damage, or diarrheal illnesses.Amoeba species, such as the giant amoeba (ranging in size from 1-5 mm) and the more lethal Naegleria fowleri (causing mortal cerebral infections) all share their amorphous shape in common. Or, I suppose another way to say that is that they share a lack of any fixed body structures.

These heterotrophs ‘ooze’ forwards, using something called cytoplasmic flow, which actively alters the viscosity, or thickness, of their cytoplasm. This enables them to sort of roll forward, like a beach ball partially filled with water.


The animal-like protozoan category isn’t reserved exclusively for heterotrophs – there are actually some organisms, called mixotrophs that use a mixture of heterotrophic and autotrophic feeding strategies. Mixotrophic protists can both hunt bacteria or smaller protists (and so fall under the umbrella term animal-like) and, in the presence of light, can photosynthesize. One common variety of this interesting protist is Euglena, which also happens to be a flagellated protist. There are about 800 different species of Euglena, each with this capacity to hunt as well as use their chloroplasts for photosynthesis – talk about dining options.

Lesson Summary

Protists are a kingdom of solitary or colonial living single-celled eukaryotic organisms (having membrane bound organelles) that can’t be classified as belonging to the plant, animal, bacteria, or fungi kingdoms.

Animal-like protists are heterotrophic, meaning they acquire nutrients by hunting and consuming bacteria and other smaller protists. Some animal-like protists are ciliated (covered in hair-like cilia projections), while others are flagellated, having one or more tail-like projections, called flagellum, while still others are amorphous and utilize cytoplasmic flow to roll forward by altering the viscosity of their cytoplasm. Mixotrophic protists, use a mixture of heterotrophic and autotrophic feeding strategies.


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