Ciliates

Ciliates are a diverse group of single‑celled eukaryotes in the phylum Ciliophora, within the larger group Alveolata. They are characterized by numerous cilia covering the cell surface or organized in distinct patterns, which beat in coordinated waves to propel the cell and to direct food toward the oral region. Many ciliates also possess attachment structures and complex cortexe that support their movement and feeding.

All ciliates have two kinds of nuclei. A micronucleus acts as a germline for genetic transmission, while a macronucleus controls daily metabolic and developmental functions. Reproduction is primarily asexual, typically by transverse or longitudinal binary fission; many species can also undergo conjugation, a genetic exchange process that increases variation and can repair DNA.

Feeding is mediated by ciliary action that generates currents, delivering food to a cytostome or oral groove where food vacuoles form. Diets are varied, including bacteria, algae, and other small protists; some ciliates host photosynthetic endosymbionts, and others are parasites. Many carry specialized organelles such as trichocysts or toxicysts for defense or prey capture, and contractile vacuoles that regulate osmotic balance, especially in freshwater species.

Ecologically, ciliates inhabit freshwater, marine, and soil environments, and are common in the guts of animals such as termites and ruminants. Some species are free-living, while others are symbionts or parasites, including Balantidium coli in humans. The group is taxonomically diverse, with several major classes recognized within Ciliophora.