Index to this page

The Protists

The Origin and Evolutionary Relationships of the Protists (Protista)

What are protists?

The cladogram also shows the protistan branches that led to the plants, fungi, and animals.

Protists without mitochondria

There are several groups of protists that have no mitochondria. For some, this may represent a secondary loss as they evolved from ancestors that had them. For others, it seems to be a primitive condition and, if so, these organisms are close to the base of the tree of eukaryotic life (figure).

Some examples are:

Some Cellular Slime Molds (Acrasia)

The organisms in this group have a complex life cycle during the course of which they go through unicellular, multicellular, funguslike (form spores) and protozoanlike (amoeboid) stages.

Thousands of individual amoebalike cells aggregate into a slimy mass. The aggregating cells are attracted to each other by the cyclic AMP (cAMP) that they release.

With the exception of one species that causes powdery scab on potatoes, these organisms are of little economic importance.

The Euglenozoa

Ciliates, Sporozoans, and Dinoflagellates


All of this rightly suggests that although they are unicellular, there is nothing rudimentary about the ciliates. Their single cell is far more elaborate in its organization than any cell out of which multicellular organisms are made.


All the sporozoans are parasitic. They lack the power of locomotion during most (in some cases, all) of their life cycle.

The genus Plasmodium causes malaria, one of the greatest scourges of humans. Malaria has probably caused more human deaths than any other infectious disease; even today it is estimated to kill a million people a year in the sub-Saharan Africa.

The organism is transmitted from human to human through the bite of mosquitoes of the genus Anopheles.

The diagram shows the life cycle of Plasmodium vivax.

Most forms of malaria are chronic. The organisms may coexist with their host for years (but cannot complete their life cycle there).
How they evade the immune response of their host.


Diatoms, Golden Algae, Brown Algae, and Water Molds

The first three members of this clade share:


Diatoms are unicellular. Their cell wall or shell is made of two overlapping halves. These are impregnated with silica and often beautifully ornamented. The photo (courtesy of Turtox) is of Arachnoidiscus ehrenbergi magnified some 400 times.

Diatoms are the main producers in aquatic environments; that is, they are responsible for the majority of the photosynthesis that occurs in fresh water and in the oceans. They serve as the main base of the food chains in these habitats, supplying calories to heterotrophic protists and small animals. These, in turn, feed larger animals.

Golden Algae (Chrysophyta)

Brown Algae (Phaeophyta)

Water Molds (Oomycetes)

As their name suggests, water molds were once considered to be fungi. But unlike fungi, the cell wall of water molds is made of cellulose, not chitin. Furthermore, their gene sequences are very different from those of fungi (and most closely related to those of brown algae).

Some notable water molds:

Red Algae


Plants, including the green algae (Chlorophyta) are described in a separate page. Link to it.

Plasmodial Slime Molds (Myxomycetes)

At one stage in their life cycle, these organisms consist of a spreading, slimy mass called a plasmodium that moves slowly over its substrate (e.g., a rotting log) engulfing food and growing as it does so.

Eventually, the plasmodium develops stalks that produce and release spores. If the spores land in a suitable location, they germinate forming single cells that move by both flagella and pseudopodia. These fuse in pairs and start forming a new plasmodium.

The left photo (courtesy of Prof. I. K. Ross) shows the plasmodial stage of Stemonitis just before it formed sporangia. The right photo (courtesy of Turtox) shows the fully developed sporangia of Stemonitis.

Physarum polycephalum, another member of this group, is the subject of many laboratory studies.

External Link
PhysarumPlus - An Internet Resource for Students of Physarum polycephalum
and Other Acellular Slime Molds
(Please let me know by e-mail if you find a broken link in my pages.)


The fungi are discussed in a page of their own. Link to it.

Metazoa (Animals)

The metazoa are described in two separate pages:
Review the evolutionary relationships of the various clades of protists.
External Link
A portal into the various groups of protists (and other eukaryotes) with many illustrations in color.
(Please let me know by e-mail if you find a broken link in my pages.)
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13 April 2001