Kingdom Protista

The Kingdom Protista includes with a few exceptions, all the unicellular eukaryotic micro-organisms. They are predominantly aquatic where they occur as solitary cells. Some of them may form colonies consisting of similar cells. The protists are regarded as ancestors of all multicellular eukaryotic organisms.

Most of the protists are free living and inhabit the surface of water bodies constituting the planktons, both phytoplanktons and zooplanktons.

Many of the protists are capable of locomotion. It may occur by wriggling or mucilage propulsion mechanisms or it may be with the help of locomotor structures such as pseudopodia, flagella and cilia.

The members of this kingdom show a diverse mode of nutrition. They may be autotrophic or heterotrophic. Heterotrophic forms may be holozoic or parasitic or saprophytic or even mixotrophic (obtaining nourishment by more than one method).

The most common mode of reproduction is asexual. It may occur by binary fission, or multiple fission or budding or spore formation. Sexual reproduction involves meiosis and syngamy (fertilization).

Protists can be broadly distinguished into three major groups, namely

1. Photosynthetic protists

2. Slime moulds and

3. Protozoan protists