A single-celled paramecium backs up and tries again when it hits something
Bump a paramecium into an obstacle and the hairlike cilia covering it reverse their power stroke, sending it swimming backwards for a moment before it sets off again at a new angle. If it hits the object once more, it repeats the manoeuvre until it gets past. Biologists call this the avoidance reaction.
Paramecium is a genus of single-celled ciliates found in fresh, brackish and salt water, often thriving in stagnant ponds. Cells measure between 0.06 and 0.3 millimetres and are shaped like eggs, cigars or feet. Because some species are easy to grow and readily divide or mate on cue, they have become the lab rats of their group, a fixture of classrooms and research labs.
Movement comes from rows of cilia covering the body. Each hair beats with a fast, stiff power stroke, then curls and sweeps forward slowly to reset. Coordinated waves ripple across this carpet, an effect likened to wind rolling over a grain field, and the cell corkscrews as it swims. Food is swept by constantly beating cilia along a deep groove running to the middle of the cell. Most species eat bacteria and other tiny organisms, but a few also harbour green algae inside them that supply some nutrients. Contractile vacuoles pump out the water that keeps seeping in.
The outer covering, the pellicle, is stiff yet elastic, patterned with tiny polygons each pierced by a single cilium. Tucked between its layers in most species are trichocysts, spindle-shaped organelles that fire thin, harmless filaments, often in defence.
The organism has a long scientific pedigree. Christiaan Huygens described it clearly in a letter in 1678, and an anonymous drawing appeared in the Royal Society's journal in 1703. In 1718 the French microscopist Louis Joblot found one in oak-bark infusion and named it the slipper, a nickname that lasted two centuries. John Hill coined Paramecium, from the Greek for oblong, in 1752. Later scholars fiddled with the spelling until C. G. Ehrenberg restored the original in 1838, and most have followed him since.
Source: Paramecium
