Kingdom Protista
These organisms are unicellular, eukaryotic and mostly aquatic. However, some are terrestrial. They are either motile or non-motile. The locomotory organs are cilia, flagella, pseudopodia, etc.
The protists are with or without cell wall. If cell wall is present, it is composed of cellulose. They have membrane-bound cell organelles such as mitochondria, chloroplasts, ER, Golgi complex, etc. The nucleus is well-organized i.e. with nuclear envelope, nucleoplasm, nucleolus and helical DNA with histone proteins forming chromosomes. They show either photosynthetic, holozoic, saprophytic or parasitic mode of nutrition. The reserve food material is in the form of glycogen or starch and fat.
The reproduction is by asexual or sexual method. Asexual reproduction occurs by mitotic or amitotic cell division while sexual reproduction is by fusion of male and female gametes forming zygote. The zygote directly develops into young individual without formation of embryo.
Protists may further be distinguished into :
a) Plant like protists - They are autotrophic (photosynthetic), have cellulosic cell wall and store reserve food as starch e.g. dinoflagellates, diatoms and desmids (golden algae). Diatoms and desmids are together grouped as chrysophytes. The cell wall of diatoms is composed of two overlapping halves resembling a soap case. The remains of their cell walls is called diatomaceous earth which is used in chromatography, filtration and polishing. They are chief produces of aquatic envitonment.
b) Animal like protists - These are heterotrophic, ingest food, lack cell wall and store reserve food as hydrogen e.g. Amoeba, paramoecium.
c) Fungi like protists - They are saprophytic and lack cell wall e.g. Slime moulds.
d) Euglenoids - These are primarily photosynthetic aquatic organisms. However, they behave as heterotrophs in the absence of light. e.g. Euglena
Thus Kingdom Protista is a link between prokaryotic Kingdom Monera and complex eukaryotic Kingdoms-Plantae, Fungi and Animalia.