Jon Robb, age 12, of Lake Zurich, Illinois, for his question:

How is the paramecium classified?

The paramecium is a prototzoan. Roughly 80,000 species of protozoa have been classified, all of them single celled animals and most of them microscopic in size. All of them move around and their different methods of locomotion help to classify them in separate groups. Also, some have more rigid shapes and working systems. The paramecium belongs in this more specialized group.

All single celled animals belong in the Phylum Protozoa. Perhaps the simplest protozoan is the amoeba, a mobile microscopic blob of cytoplasm. He is classified in a group called Rhizopoda, or Sarcodina. Both these terms refer to the temporary feet he uses to get from here to there. All species of the enormous phylum are subdivided into classes and subclass based on locomotion and other basic features.

In the department of special features, the paramecium is in a class by himself. You might say that he is the royalty of the protozoa. The amoeba is a shapeless blob with a minimum of parts for special duties. The paramecium has a definite, and very elegant, permanent shape. Other permanent features include a front and rear end, a mouth and gullet and what well may be nature's first try at creating a nervous system.

The surface of his dainty cell is covered with furry little fibers called cilia. Below the surface, a neat system of fibers moves the cilialn rhythmical waves. This is how he swims briskly forward, stops when he bumps into an obstacle, backs off and veers in another direction. The waves move obliquely, so that the moving midget twirls around as he goes. Since the cilia controls his locomotion, the paramecium is a ciliate of the Class Ciliata. The common species P. caudatum has as many as 25,000 cilia.

The mobile midget, alias the slipper animalicule, is shaped like a slender little slipper. The pointed toe is the rear end, with an anal pore that jets out solid wastes. The round heel of his slipper heads him forward. This end of his cell has an oral pore. Here the ciliated surface folds inside, forming a pocket with a mouth and gullet. The lining of cilia make miniature waves to wash down bacteria, algae and other floating scraps. They also whirl each snack into a digestible wad, which circulates around inside as a food vacuole.

The dozen or so major paramecium species range in size from five to 5,000 microns. About 1,000 of the smallest ones, lined up toe to heel, measure an inch. Some species have rounder toes, fewer cilia or other slight variations. But all of them are ciliates of the Class Ciliata.

Paramecia are very popular subjects in the study of microscopic biology. Hence we know quite a lot about them. The miniature details of their cell structures are fascinating. What's more, these protozoa have features that suggest the biological system of more advanced animals. Their cilia are moved by what resembles a primitive nervous system. Some strains practice what could be nature's earliest experiment in sexual reproduction.