Class Ciliatea

The subphylum Ciliophora has only one class, the Ciliatea. This is the largest and the most homogeneous of the protozoan classes.

The ciliophores, or ciliates, are the most highly specialized and complicated of the Protozoa, and indeed probably represent the most complex of all living cells. They are characterized by cilia. The rows of cilia are called cirri, which can be used for walking or jumping. Cilia, membranelles, and cirri move in a coordinated fashion. Most ciliates possess a cell mouth or cystosome.

Most ciliates live in freshwater or seawater. They do not have any parts to help them move. Instead, they move by changing shape. They move by means of many short hair-like parts called cilia.

Perhaps the best known ciliate is the Paramecium. It is shaped like the sole of a slipper. It has a long groove on one side. The groove is lined with cilia that help pull food along the groove and into the mouth-like gullet. At the end of the gullet, a bubble-like food vacuole forms around the food. When the vacuole is full, it carries the food into the cell where the food is broken down

Cilia have the same structure as do flagella; they differ from flagella chiefly in that they are generally more numerous and are considerably shorter. Compound ciliary organelles, evolved from the adhesion of varying numbers of individual cilia, are of common occurrence.

Some ciliates also have myonems, contractile threads. All have a complex skin, the cortex, which includes the cell membrane. In some groups, the cortex contains small barbs known as trichocysts, which are discharged when the cell is stimulated in certain ways. The ciliates have another unusual feature: they have two kinds of nuclei, macronuclei and micronuclei. One or more of each kind is present in all cells. They also have a complex system for exchange of genetic information, in which cells conjugate and the macronuclei undergo meiosis. The macronucleus in certain ciliates contains 50 to 100 times as much DNA as the micronucleus and so is believed to represent multiple copies of it.

The body shape is usually constant and in general is asymmetrical. The ciliate body is typically covered by a complex, living pellicle, usually containing a number of different organelles.

Mucigenic bodies are the group of pellicular organelles found in many ciliates. They are arranged in rows like trichocysts and discharge a mucoid material that may function in the formation of cysts or protective coverings.

About 6,000 species of ciliates are known, both freshwater and saltwater forms. Almost all are free-living (non-parasitic).

Date: 2014-12-22; view: 1076