|"Ciliata" from Ernst Haeckel's Kunstformen der Natur, 1904|
The ciliates are one of the most important groups of protists, common almost everywhere there is water — lakes, ponds, oceans, and soils, with many ectosymbiotic and endosymbiotic members, as well as some obligate and opportunistic parasites included. Ciliates tend to be large protozoa, a few reaching 2 mm in length, and are some of the most complex in structure. The name ciliate comes from the presence of hair-like organelles called cilia, which are identical in structure to flagella but typically shorter and present in much larger numbers with a different undulating pattern than flagella. Cilia occur in all members of the group (although the peculiar suctoria only have them for part of the life-cycle) and are variously used in swimming, crawling, attachment, feeding, and sensation.
Unlike other eukaryotes, ciliates have two different sorts of nuclei: a small, diploid micronucleus (reproduction), and a large, polyploid macronucleus (general cell regulation). The latter is generated from the micronucleus by amplification of the genome and heavy editing. Division of the macronucleus occurs by amitosis, the segregation of the chromosomes is by a process, whose mechanism is unknown. This process is by no means perfect, and after about 200 generations the cell shows signs of aging. Periodically the macronuclei must be regenerated from the micronuclei. In most, this occurs during conjugation. Here two cells line up, the micronuclei undergo meiosis, some of the haploid daughters are exchanged and then fuse to form new micronuclei and macronuclei.
With a few exceptions, there is a distinct cytostome or mouth where ingestion takes place. Food vacuoles are formed through phagocytosis and typically follow a particular path through the cell as their contents are digested and broken down via lysosomes so the substances the vacuole contains are then small enough to diffuse through the membrane of the food vacuole into the cell. Anything left in the food vacuole by the time it reaches the cytoproct (anus) is discharged via exocytosis. Most ciliates also have one or more prominent contractile vacuoles, which collect water and expel it from the cell to maintain osmotic pressure, or in some function to maintain ionic balance. These often have a distinctive star-shape, with each point being a collecting tube.
Most ciliates feed on smaller organisms (heterotrophic), such as bacteria and algae, and detritus swept into the mouth by modified oral cilia. These usually include a series of membranelles to the left of the mouth and a paroral membrane to its right, both of which arise from polykinetids, groups of many cilia together with associated structures. This varies considerably, however. Some ciliates are mouthless and feed by absorption, while others are predatory and feed on other protozoa and in particular on other ciliates. This includes the suctoria, which feed through several specialized tentacles.
In some forms there are also body polykinetids, for instance, among the spirotrichs where they generally form bristles called cirri. More often body cilia are arranged in mono- and dikinetids, which respectively include one and two kinetosomes (basal bodies), each of which may support a cilium. These are arranged into rows called kineties, which run from the anterior to posterior of the cell. The body and oral kinetids make up the infraciliature, an organization unique to the ciliates and important in their classification, and include various fibrils and microtubules involved in coordinating the cilia.
The infraciliature is one of the main component of the cell cortex. Another are the alveoli, small vesicles under the cell membrane that are packed against it to form a pellicle maintaining the cell's shape, which varies from flexible and contractile to rigid. Numerous mitochondria and extrusomes are also generally present. The presence of alveoli, the structure of the cilia, the form of mitosis and various other details indicate a close relationship between the ciliates, Apicomplexa, and dinoflagellates. These superficially dissimilar groups make up the alveolates.
- Class Karyorelictea
- Class Heterotrichea (e.g. Stentor)
- Class Spirotrichea
- Class Litostomatea
- Class Phyllopharyngea
- Class Nassophorea
- Class Colpodea (e.g. Colpoda)
- Class Prostomatea (e.g. Coleps)
- Class Oligohymenophorea
- Class Plagiopylea