The Protista encompass the group of organisms that are harder to classify. These (usually) single celled organisms are hard to fit into the typical plant or animal types. As a result, there are three main subkingdoms of the Protista kingdom, protophyta, protozoa, and gymnomycota. The protophyta are more plant like, while the protozoa are more animal like. There is mismatching and crossing over however. For example, the Euglena is in the protozoan subkingdom, but has chloroplasts, which are characteristic of plant cells. Finally, you have the Gymnomycota, the area where they dump the harder to classify of the harder to classify.

Subkingdoms:

-Protozoa
-Protophyta
-Gymnomycota


It should also be noted that the phyla mentioned in the above node are part of the subkingdom Protozoa.

The kingdom protista is the most diverse of all kindoms. It is composed of microscopic eukaryotic organisms, which are often hard to put in another kingdom. It is divided into 3 categories:

1. Protozoans
2. Algae
3. Slime Molds and Downey Mildew

1. The genus protozoa is composed of those protists who are most like animals. It is divided further into:

Ciliophora: protists who move using cilia, tiny hair like structures.
Zoomastigina: protists who move using flagella, longer whip like structures
Sarcodina: protists who move using pseudopodia, extentions of their cytoplasm
Sporozoa: protists who are non-motile and generally live as parasites

2. Algae are protists that are most like plants. They are divided as follows:

Euglenophyta: special protists that are both autotrophic and heterotrophic and use euglenoid movement.

Baciliariophyta: protists with a Silicon Dioxide shell which make up most phytophlankton. ex. Diatoms
Dinoflagellates: protists with a cellulose shell which use flagella to move.

Chlorophyta: green algae w/ pigment chlorophyll
Rhadophta: red algae w/ pigment phycobilin
Phaophyta: brown algae w/ pigment fucoxanthin

3. Slime molds and downey mildew are the really strange of the really strange. They are divided as follows:

Myxomycota: cellular slime molds
Acrasiomycota: plasmodial slime molds

Oomycota: downey mildews and water molds

A good place to get pictures of protists is: http://mac2031.fujimi.hosei.ac.jp/PDB/Images/menuE.html

The kingdom Protista started out in the three kingdom system to "solve" the problems of the two-kingdom system of animalia (animals) and plantae (plants). Bacteria, eukaryotic single celled organisms, and fungi are not quite plants or animals, so the two-kingdom system was not really effective. With the three-kingdom system, anything that wasn't quite a plant or an animal was put in kingdom protista. This wasn't much help either, as bacteria, fungi, and single-celled eukaryotes are still quite different from eachother. So a five-kingdom system was created, consisting of animalia, plantae, fungi, monera (bacteria), and protista. This solved many of the problems with the two and three kingdom systems, and is the system in wide use today. Protista still remains a quite inclusive kingdom, as it includes a diverse selection of organisms.

In the five-kingdom system, protists have the following characteristics:

  • Eukaryotic cell(s)
  • Usually unicellular (seaweeds and related organisms are multicellular)
  • Usually reproduce asexually (although sexual reproduction is possible for some species)
  • Can be either autotrophic or heterotrophic
  • They live in aquatic environments
  • The fields of biological classification and phylogenetics are in a state of upheaval. It was very natural for biologists to group plants, animals, and fungi into separate eukaryotic kingdoms, and to throw all the little eukaroytes that didn't seem to fit into the kingdom Protista. However, advancements in microscopy and comparative genetics have given biologists a much stronger understanding of the evolutionary tree. It is now clear that the kingdom Protista is not a true clade.

    A clade is defined as a group of organisms that contains a most recent common ancestor and all of its descendants. The adjective used to label a group a clade is monophyletic. Animalia, Plantae, and Fungi are all monophyletic. Protista fails to meet both criteria for a clade. For example, biologists have agreed that the protist Microsporidia evolved from Fungi, but the protist Myxozoa evolved from Animalia. Furthermore, animals seem to have evolved from the protist Choanoflagellates. A group of organisms like the protists that meets neither criterion for a clade is termed polyphyletic.

    Biologists aim for a comprehensive hierarchical classification system based on clades. It will take a while for phylogeneticists to sort through the evolutionary chain, but they are making progress. A new monophyletic kingdom Chromista has been agreed upon and more are to come. Some protists will be grouped into the new kingdoms and others will be grouped into Plantae, Metazoa (Animalia), or Fungi. In the meantime, the group Protista is a collection of tiny eukaryotic organisms that we don't know where to put.

    "Phyla" of "kingdom" Protista

    Not complete, and of course non-monophyletic.

    References:

    • http://tolweb.org/tree/
    • http://www.ucmp.berkeley.edu/alllife/eukaryota.html

    Pro*tis"ta (?), n. pl. [NL., fr. Gr. prw`tistos first.] Zool.

    A provisional group in which are placed a number of low microscopic organisms of doubtful nature. Some are probably plants, others animals.

     

    © Webster 1913.

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