Ciliate
\nThe
ciliates are one of the most important groups of
protists, common almost everywhere there is water -
lakes,
ponds,
oceans, and soils, with many
ecto- and
endosymbiotic members. 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. 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: large,
polyploid macronuclei, and smaller,
diploid micronuclei. Speaking very loosely, the former is more important for protein synthesis, and the latter for genetics. Periodically the macronuclei must be regenerated from the micronuclei. In most, this occurs during
sexual reproduction, which is not usually through
syngamy but through
conjugation. Here two cells line up, the micronuclei undergo
meiosis, some of the
haploid daughters are exchanged and then fuse to form new micro- 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 absorbed, then are discharged at a point called the
cytoproct. 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 in maintaining ionic balance. These often have a distinctive star-shape, where the points are the collecting tubes.
Most ciliates feed on smaller organisms, 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.
Related topics
\n*Nuclear Dimorphism
