Introduction to the Ciliates

Ciliates are all single-celled protists and, therefore, carry out all of their life functions within the confines of a eukaryotic cell. These cells are composed of the same basic parts found in all eukaryotic cells, but these parts (called organelles) are generally more complex than the cells that make up the tissues of plants and animals. The purpose of this section is to introduce you to the ciliate cell.

Distinguishing Features of Ciliates

Ciliates have several distinguishing features and they are one of the most clearly defined groups of protist. These features are:

1. Cilia at some stage during the life cycle.

Cilia are fine hair-like organelles that beat rhythmically for locomotion or to create feeding currents. Body or somatic cilia lie in rows called kineties (sing. kinety). Oral cilia aid in food capture and may be simple or form complex compound stuctures.

2. The cilia are anchored within the cell with three unique fibers, called the Infraciliature.

The Infraciliature consists of a ribbon transverse microtubules, a ribbon of postciliary microtubules, and a striated fibrillar kinetodemal fiber.

To see an illustration of the infraciliature, click HERE.

In addition to these three fibers, the kinetosome may also be associated with:

parasomal sacs - An invagination of the cell membrane near the cilium which may function in endocytosis. Parasomal sacs may be homologous to the micropore of Apicomplexa.
subkinetal microtubules - microtubules that run underneath the kinety.

3. There are two types of nuclei: a macronucleus and a micronucleus. The macronucleus is vegetative and is responsible for protein transcription (in other words the day-today operations of the cell). The micronucleus is smaller and primarily functions in exchange of genetic material during conjugation.

4. The fission line of cell division cuts at right angles to the kineties, dividing the mother cell into an anterior proter and a posterior opisthe. (In contrast, most other protists with bilateral symmetry divide longitudinally, rather than transversely).

During cell division, the oral structure of the opiste must be formed and it does so in a process called stomatogenesis. Stomatogenesis varies somewhat from group to group in the ciliates.

5. Genetic recombination is by conjugation. Most organisms have gametic genetic recombination. Ciliates have a unique process called conjugation in which micronuclei are exchanged.

To see an illustration of the conjugation, click HERE.]

6. Food is ingested through a permanent opening called a cytostome where it is enclosed in a food vacuole.

There is a great deal of variability in the oral structures of ciliates. In many ciliates, the cytostome lies at the base of a cytopharynx - a tube supported by microtubules. These microtubules may form thick rods (called nematodesmata) so that the cytopharynx looks like a basket. Other ciliates, specifically the suctoria, have multiple cytostomes at the tips of feeding tentacles. Still other ciliates have the cytostome-cytopharynx tucked into a cell indentation (called a vestibule or infundibulum) to make filter feeding more efficient.

[Show a picture of these three types of mouths using peritrich, suctoria, and prorodon]

Some Euglenoids and Dinoflagellates also have cytostomes, but they are not as elaborate as those found in ciliates.

7. Contractile Vacuoles - an osmoregulatory structure consisting of a membrane bound vessicle associated with microtubular ribbons.

8. Cytoproct - A permanently present slit-like structure from which undigested material is extruded (it is effectively the cell anus). The cytoproct is usually located in the posterior part of the cell.

9. Extrusomes (=extrusive organelle) - exocytotic vesicles that are able to extrude their contents are found in many types of protists. While they are known to have different functions depending upon their type, they all have one festure in common: they discharge, sometime explosively, their contents outside the cell when subjected to chemical or mechanical stimuli. Several types of extrusomes are found in ciliates, including toxocysts, trichocysts, haptocysts, and mucocysts.

10. Alveolar Membrane System - consists of membrane-bound sacs lying just beneath the plasma membrane. Alveoli are also been found in dinoflagelates and apicomplexa, and this feature is considered an synapomorphy uniting these three phyla.