Protista (Protistos = Primary) includes unicellular eukaryotes and show the following characters :
(1) Protists include solitary unicellular or colonial unicellular eukaryotic organisms which not form tissues.
(2) The unicells may be naked or covered by cell wall, pellicle, cuticle or shell.
(3) Simple multinucleate organisms or stages of life cycles occur in a number of groups.
(4) The organisms possess double and porous nuclear membranes, mitochondria, golgibody, plastids (in many), vacuoles, lysosomes and ribosomes is also present. Centrosome is occur in many cases.
(5) In many forms, plastids, (9+2 strand) flagella and other organelles are present.
(6) Some protists possess contractile vacuole for regulation of their water content.
(7) Their reproductive cycles typically include both asexual divisions of haploid forms and true sexual processes with karyogamy and meiosis.
(8) The organisms move by flagella or by other means or are non-motile.
(9) It may be photosynthetic, holotrophic, saprotrophic, parasitic and symbionts. Some have mixotrophic nutrition (holotrophic + saprobic). The photosynthetic, floating protists are collectively called phytoplankton. The free-floating, holozoic protozoans are collectively termed zooplankton.
(10) Asexual reproduction is the most common method in protists. It involve binary fission (Paramecium, Euglena, Amoeba), multiple fission (Amoeba), plastotomy (Opalina), budding (Paracineta, Arcella) and spore formation (Slime moulds).
(11) Sexual reproduction is believed to have originated in primitive protists. It involve isogamy (Monocystis), anisogamy (e.g., Ceratium) and oogamy (e.g., Plasmodium).
(12) Unicellular protists have been broadly divided in to three major groups :
Photosynthetic protists : e.g., Dinoflagellates, Diatoms, Euglenoids.
Consumer protists : e.g., Slime moulds or Myxomycetes.
Protozoan protists : e.g., Zooflagellata, Sarcodina, Sporozoa, Ciliata.