Click for larger image

Introduction
Paramecium caudatum is a species of free-living protozoan of the order Hymenostomatida and kingdom Protista. Paramecia live in ponds and slow-moving streams and can be cultivated easily in the laboratory. Although they vary in size, most Paramecium species are about the size of the period at the end of this sentence. The basic shape varies, depending on the species: Paramecium caudatum is elongated and gracefully streamlined while Paramecium bursaria resembles a footprint. The term paramecium is also used to refer to individual organisms in a Paramecium species.

Paramecia are among the most complex single-celled organisms. They swim rapidly, usually in a corkscrew fashion, by means of coordinated, wavelike beats of their many short, hairlike projections, called cilia. Paramecia feed on smaller organisms, such as bacteria.

A thin layer of clear, firm cytoplasm (ectoplasm) lies directly beneath the flexible body membrane (pellicle) and encloses the inner, more fluid portion of the cytoplasm (endoplasm), which contains granules, food vacuoles, and crystals of different sizes. Paramecia have one large nucleus and one or more smaller nuclei. A network of fibers below the cell surface connects the cilia. Embedded in the ectoplasm are spindle-shaped bodies (trichocysts) that may be released by chemical, electrical, or mechanical means. Originally believed to be a defense reaction, they appear to be extruded as a reaction to injury or for use as an anchoring device.

Diagram

Feeding
Paramecium has a permanent feeding mechanism, consisting of an oral groove on the ventral surface which runs diagonally posterior to the mouth and funnel-shaped gullet into which food is drawn by the combined action of cilia which cover the body and other cilia lining the gullet.

They feed on small organisms such as bacteria and even other smaller protozoa. Food in the gullet forms a ball which passes into the protoplasm as a food vacuole. The food is digested as the vacuole passes through the organism, and the waste is passed out from a special place called the anal pore.

Contractile Vacuoles
Two, occasionally three, star-shaped spots that seem to appear and disappear close to the surface near the ends of the cell in the paramecium are the contractile vacuoles. They function in regulating the water content within the cell and may also be considered excretory structures since the expelled water contains metabolic wastes.

Cilia and Ciliary Motion
Paramecium, like other ciliates, are covered by hair-like organelles that move in a coordinated manner to propel the individual and to direct bacteria and other food particles into their mouths. A cilium (pl., cilia) is an extension of the cell membrane containing an arrangement of microtubules attached to a basal body, which is anchored by cytoskeletal proteins. They are similar to flagella, the main differences being length and nature of their motion. The cytoplasm extends out into the cilium itself. The organelle is not simply a 'hair' that is attached to the cell.

The speed of motion is about four times its own length per second. It moves so quickly that microscopists have to add a thickening agent to the water to slow it down to study it.

As the paramecium moves through the water it rotates on its axis and small particles of debris and food are collected and swept into the gullet. If a paramecium comes across an obstacle, it stops, reverses the beating of the cilia, swims backwards, turns through an angle and moves forward again on a slightly different course.

Behavior - or How does the Paramecium change direction?
The motion of an individual cilium superficially resembles that of an oar, in that it sweeps through the medium with a power stroke that propels the cell in the opposite direction. Each power stroke and return stroke involves perhaps thousands of chemical reactions. There may be dozens of strokes per second, and perhaps thousands of cilia. Paramecia respond very quickly to obstacles or changes in their environment.

All of the cilia have a calcium-activated reverse gear; when a cell bumps into an obstacle, calcium channels open, the cell depolarizes. The voltage normally present across the outer membrane leaks away, as ions rush through open channels to even out the across membrane charges, and the ciliary machinery switch gears, causing the cilia to switch into the opposite motion and the cell backs up fast.

Normally not all the cilia are swimming forward during forward swimming; some are moving in reverse mode. When the number of cilia that are swimming forward exceeds the cilia that are in reverse mode, the cell moves forward. When the cell re-establishes cell polarization, the cilia return to their forward movement. This doesn't happen quite smoothly, and when the paramecium resumes its journey, it is usually in a slightly different direction than before - giving the apearance that it has made a detour around the obstacle.

What is the significance of this? Galvanotaxis provides a neat model of how our very own nerve cells communicate, via voltage gated channels. Cells are stimulated or not by the voltage gradient across their outer membrane; when they are stimulated, they depolarize, meaning ions like potassium and chloride rush through pores or channels that can open when the voltage exceeds a certain level, i.e. they are "voltage gated").

Nuclei
Paramecia have two kinds of nuclei: a large ellipsoidal nucleus called a macronucleus and one or more small nuclei called micronuclei. Both types of nuclei contain the full complement of genes that bear the hereditary information of the organism. The organism cannot survive without the macronucleus; it cannot reproduce without the micronucleus. The macronucleus is the centre of all metabolic activities of the organism. The micronucleus is a storage site for the genetic material of the organism. It gives rise to the macronucleus and is responsible for the genetic reorganization that occurs during conjugation (cross-fertilization).

Reproduction
Paramecium has two means of reproduction, simple division and conjugation. 

Division. In favourable conditions the cell divides in two by a process called binary fission. The nuclei divide, the rear half develops a new gullet, and the front half grows a new anal pore. Then the paramecium breaks into two individuals. This whole process make take place two or three times a day if conditions are right.

Conjugation. This is a more complicated method. It involves two cells coming together to exchange nuclear material. Two compatible cells join and part of their cytoplasm fuses. They do not need their big macronucleus and it is slowly destroyed. Their micronucleus goes through a long process where its chromosomes are reduced in number by half (a process called meiosis) and then the post-meiosis nucleus replicates, so each of the mating cells has an extra nucleus. Then, each cell trades the extra nucleus with each other. Now both have a pair of "his and hers" nuclei, which fuse into one, mixing the chromosomes together. Then the one nucleus divides, and the extra one grows into the macronucleus. The two cells then separate and continue to reproduce by simple division. It is similar in some ways to sexual reproduction in more complex animals. 

Without the rejuvenating effects of conjugation, a paramecium ages and dies. Only opposite mating types, or genetically compatible organisms, can unite in conjugation. Paramecium aurelia has 34 hereditary mating types that form 16 distinct mating groups, or syngens.

Autogamy (self-fertilization) is a similar process that occurs in one animal. In cytogamy, another type of self-fertilization, two animals join together but do not undergo nuclear exchange and then reproduce by division.

More Information
For information about the care and feeding of Paramecium see:

http://www.ruf.rice.edu/~bioslabs/studies/ invertebrates/paramecium.html

After all, I guess it doesn't matter whether you look down (through a microscope) or up (through a telescope) - as long as you look.
--John Steinbeck