In the older classification, protists are classified into three groups–plantlike protists, animal-like protists, and fungus-like protists. Unlike the older classification, the modern classification identifies a protist as any eukaryote that is not an animal, a plant, or a fungus. For this reason, taxonomists grouped protists in a single taxonomic category, which is the Kingdom Protista.

The cilia move in an upward waving motion that resembles the oars on a rowboat. On the other hand, flagella are like tiny propellers that move in a whip-like manner and rotate in either clockwise or counterclockwise direction.

Photosynthetic protists are at the base of the marine food chain. They provide food and nourishment for aquatic animals, support coral reefs, and provide shelter for marine species. More importantly, they play an essential role in the diversification of aquatic life. If they are removed from the ecosystem, the food chain would be affected. The organisms that feed on these protists would die and it would result in the disruption of balance in the ecosystem.

The tree of life illustrates how the major groups of organisms are related to each other. In this diagram, the protists are grouped into five clades that are not monophyletic. This means that the organisms are not closely related.

On the other hand, the cladogram shown in figure 21-17 shows that the protists are divided into six major clades, wherein animals and fungi emerged from a protist ancestor. The problem in this representation is that each clade is more related to other plants, animals, and fungi when compared to the other protists. This causes confusion as to where the kingdoms Plantae, Animalia, and Fungi really fit into these clades.

Paramecia reproduce asexually using binary fission. This is the predominant way of reproducing offspring in all paramecium strains. In this process, the cell divides into two daughter cells, wherein each of them receives half of the cell organelles from the parent cell. This process is called mitosis.

However, when a paramecium suffers stress under unfavorable environmental conditions, it can reproduce sexually through conjugation, wherein two organisms undergo meiosis and produce four haploid nuclei. This process allows the paramecium to exchange genetic information. Eventually, after conjugating, they reproduce using mitosis.