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What is the smallest animal in the world?

The smallest animals are also the first living creatures at the bottom of the animal ladder. Just as a person's life begins with that of one small cell about 1/300 inch (0.1mm) across, the animal kingdom starts with the appearance of of single-cell creatures whose size varies between a few thousandths of an inch and 1/25 inch (1 mm): the protozoa.

In the past there were numerous arguments between botanists plant specialists and zoologists (animal specialists) about some of the protozoa. Certain species do in fact have characteristics belonging to both kingdoms; it is as if, at the beginning of this split in living organisms, there had been some hesitation in the choice of the form of life, whether to be animals or plants.

But the paramecium, known as the slipper animalcule, is without any doubt an animal, measuring about 1/100 inch (0.2mm.). What is a relatively large size for a single-cell animal is minute for an insect. And yet this is also the size of a caddis fly with its four bristly wings, and of a parasite wasp; it is also that of a shell-fish, a kind of sand-hopper.

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