What are small, one-celled animals such as amoebae and ciliates classified as?

Small, one-celled animals such as amoebae and ciliates are classified as protozoa. This classification is based on their characteristics, which include being unicellular eukaryotes that often exhibit animal-like behaviors, such as motility and the ability to ingest food. Protozoa play essential roles in various ecosystems, serving as both predators in microbial food webs and as indicators of environmental health.

In contrast, bacteria are prokaryotic organisms that differ significantly in structure and function from protozoa. Fungi represent a separate kingdom and are primarily decomposers, feeding on organic matter, while algae are photosynthetic organisms that can be multicellular or unicellular but are fundamentally different from protozoa as they produce their own food through photosynthesis. Thus, the classification of amoebae and ciliates as protozoa reinforces the distinct characteristics and ecological roles these organisms hold within the biological classification system.