Single-celled animal of changing shape, whose very name means change.
The cytoplasm is differentiated into an inner, granular, more fluid endoplasm and an outer, clear, firmer ectoplasm, but even this structure is dynamic, for the two types are interconvertible. In a moving amoeba, the ectoplasm in a given area of the periphery will suddenly become fluid, permitting the endoplasm to flow forward into a projecting lobe, called a pseudopod ("false foot"). Near the tip of the pseudopod, the endoplasm flows to the sides and becomes firmer, becomes ectoplasm, so that the pseudopod extends as a tube of firm ectoplasm with endoplasm inside. The animal can progress at about 2 centimeters per hour at this pace, hardly racing along, but moving nonetheless. (They don't usually go in straight lines, though, having little sense of what we would call purpose.)
There are stranger things yet. If you put an amoeba in a centrifuge at high speed, you can take the thing apart completely, with the various organelles (component parts) at different levels in the test tube, with the nucleus floating in there too. No surprises here: but if you leave this mixture alone, the animal will re-assemble itself, apparently without lasting harm. (Think of the horror movies that could be made if people could do this!)
Still, there are limits. The nucleus is important. If you cut an amoeba into two pieces, one with the nucleus and one without, the one with the nucleus goes on as before, but the piece without, while it can move and feed for a time, soon becomes inactive, being unable to digest, grow or divide.
Amoebas eat by surrounding a food particle and taking it inside in a "food vacuole." Digestive enzymes (which float around inside the amoeba in small packages, ready for use) are released into the food vacuole and break the food down into usable form. Organic molecules can also be absorbed from the surrounding medium.
As everyone knows, amoebas reproduce by fission. No sexual reproduction has been observed in amoebas. Every individual, then, is, in a way, its own species, and in the same way, is immortal, never born, never dying.
There are quite a number of variations on this basic theme. There are giant amoebas such as Chaos carolinense which have about 1,000 nuclei. When such a giant divides, one daughter cell doesn't take 500, the other the other 500, as you might expect, but each nucleus divides, so each new cell gets a full set. Some amoebas live collectively for a time ("slime molds"); some have shells; they are found nearly everywhere.
We think of them as primitive, but really they're not. They've been around a lot longer than we have, and carry out every organic function we do, with a charming economy of scale, without specialized equipment, making do, enduring indefinitely.
Conventional scientific classification falls down with these things, partly because they do not breed, partly because we don't understand them very well. Every authority has a slightly different way of classifying them. All agree so far that they are in the Kingdom Protista.