Primary endosymbiogenesis occurs when a eukaryotic cell engulfs a prokaryotic cell. This prokaryotic cell then becomes an organelle in the eukaryotic cell. Mitochondria and plastids have originated in this way.
In secondary endosymbiogenesis, the product of primary endosymbiogenesis is engulfed by a eukaryotic cell. In this way a plastid can be engulfed by a eukaryotic cell.
Under the light microscope, when the chromosomal material in the nucleus shows clumps irregularly, leaving some relatively clear areas within the nucleus, the nucleus is known as vesicular nucleus.
This is characteristic of protozoan nuclei.
The Macronucleus of ciliates is described as compact because the chromatin material is more finely dispersed and clear areas cannot be observed with the light microscope.
Autotrophic nutrition -
When organisms synthesize their own organic constituents from inorganic substrates, it is known as autotrophic nutrition.
The organisms are known as Autotrophs. Autotrophic protozoa (phototrophs) use light energy to synthesize their organic molecules, but they often practice phagotrophy and osmotrophy as well.
Heterotrophic nutrition -
When organisms obtain organic molecules synthesized by other organisms, it is known as heterotrophic nutrition.
Heterotrophic protozoa may ingest their food in a soluble form or in particulate form. Particulate food is acquired by phagocytosis. Soluble food is ingested by osmotrophs or saprozoic feeders.
E.g. Amoeba , Didinium