Protists Defined

Many modern members of the Eukarya—trees, mushrooms, and dogs, not to mention ourselves—are familiar to us. We would have no problem recognizing these organisms as members of the kingdoms Plantae, Fungi, and Animalia. However, amoebas and a dazzling assortment of other eukaryotes, mostly microscopic organisms, don't fit into these three kingdoms. We call all those eukaryotes that are neither plants, animals, nor fungi protists. The pro-tists are not a clade; they are a polyphyletic group (see Figure 1.8). Some protists are more closely related to the animals than they are to other protists. Some protists are motile, while others are stationary; some are photosynthetic, while others are heterotrophic; most are unicellular, while

Plasmodium falciparum, the Malaria Parasite This stomach wall of an Anopheles mosquito is covered with cells (artificially colored blue) of a particular stage of the Plasmodium life cycle.These Plasmodium cells will give rise to cells that the mosquito can transmit to humans, causing malaria.

Mosquito Stage Plasmodium Lifecycle
(a) Peridinium sp.
Artificially Colored Animals Unicellular Protists

28.1 Three Protists (a) Most dinoflagellates are photosynthetic unicellular protists. (b) Giardia is a unicellular parasite of humans and other mammals. (c) Giant kelps are some of the world's longest organisms.

some giant kelps are not only multicellular but also huge, sometimes achieving lengths greater than that of a football field (Figure 28.1).

The protists include some of the most ancient eukaryotic organisms as well as the ancestors of the plants, animals, and fungi. The origin of the eukaryotic cell was one of the pivotal events in evolutionary history. In this chapter, we'll describe the origin and early diversification of the eukaryotes and the complexity achieved by some single cells. Then we'll explore some of the diversity of protist body forms and try to give a sense of developing current views of the evolutionary relationships of some of the protists.

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