The ciliates (Ciliophora)
The largest group of protozoans, the ciliates, are also the most complex, showing the highest level of internal organisation in any single-celled organism. Most are free-living types such as Paramecium (Figure 9.13), and as the name suggests, they are characterised by the possession of cilia, which may be present all over the cell surface or arranged in rows or bands. They beat in a co-ordinated fashion to propel the organism, or assist in the ingestion of food particles.
A unique feature of the ciliates is that they possess two distinct types of nuclei:
· macronuclei are concerned with encoding the enzymes and other proteins requiredfor the cell’s essential metabolic processes. They are polyploid, containing many copies of the genome.
· micronuclei, of which there may be as many as 80 per cell, are involved solely insexual reproduction by conjugation.
As might be expected, removal of the macronucleus leads quickly to the death of the cell; however, cells lacking micronuclei can continue to live, and reproduce asexually by binary fission.
Most ciliates possess a specialised ‘mouth’ structure, the cytostome, through which food particles are ingested (Figure 9.13). The beating of cilia directs them to a cytopharynx, a membrane-covered passage or tube, which enlarges and detaches to form a food vacuole. Fusion with lysosomes and digestion by enzymes occurs as described earlier. Undigested particles are ejected from a region on the surface (the anal pore or cytoproct).
As well as cilia, some members of the group have trichocysts projecting from the cell surface, harpoon-like structures that can be used for attachment or defence.
Some ciliated protozoans are anaerobic, such as those found in the rumen of cattle. The only ciliate known to cause disease in humans is Balantidium coli, which causes a form of dysentery.