Protozoa = First animals ("Protos" = first, "Zoon" = animal)
Protozoans were not seen until the invention of microscope (possibly as early as 1590). In 1676 Leeuwenhock first described microscopic animals and called them animal-cules. Lendermuller (1760-63) gave the name of Infusoria and kept these animals in that group.
In 1809 Lamarck included Protozoans and Rotifers in Infusoria. In 1812 Dutrochet recognized the different organization of Rotifers. Only in 1838 Ehrenberg separated Rotifers from Protozoans. In 1818 Goldfuss gave the name Protozoa. He used this name to represent the lower groups of Cuvier's. In 1845 Von Sieboki recognized the unicellular nature of the protozoans. He gave the definition of protozoa. Perty in 1852 coined the name Ciliata. In 1853 Cohen coined the name Flagellata. In 1879 Leuckart proposed the name Sporozoa. Dobell called protozoans as acellular as their body is not divided into cells. (Hyman also prefered to call them acellular).
The Protozoans are acellular animals without tissues or organs. They exist singly or in colonies. In these colonies all cells are alike. But in "Proterospongia" a colonial flagellate some degree of differentiation exists among the individuals forming the colony.
In some Protozoans all the functions are performed directly by un differential protoplasm. In some Protozoans a functional differentiation is attained. Such differentiations are carried on by organelles. They are also called organoids, or organites.
They are -
- Contractile Vacuole" for water regulation in the body.
- Photoreceptors" as sensory structures.
- Cilia and flagella" for locomotion and food collection.
- Skeletal secretions for protection and to give shape.
These organelles resemble different structures of multicellular organisms.
In Protozoans respiratory differentiation is absent. The presence of excretory mechanism is much discussed.
In Protozoans reproduction is by Asexual and Sexual methods. Asexual reproduction is by fission, budding, multiple fission etc.. Sexual reproduction is varied. The syngamy is isogamous. anisogamous and oogamous types. Some organisms show complicated life histories which include alternation of genera tions.
The phylum Protozoa includes Flagellate. Rhizopoda Ciliata and Sporozoa. Formerly Rhizopoda were considered the most primitive protozoa. But they are derived from Flagellata. This view is now universally accepted Flagellates are considered the primitive Protozoans.
The Protozoans are very small in size For example 12 sporozoan parasites (Babasia) can inhabit one Red blood cell. In a single cell several hundreds of protozoans can live. Ex Flagellate parasite " Leishmanial They are very small and are measures in microns The smallest protozoans is "2 to 3 microns and the larger ones may measure upto "250 microns" in length. Porospora gigantea a sporozoan measures 16 m.m. length.
Nearly 50, 000 protozoan species are described and probably many more are to be described.