Under the soft moss, under the stones, in all localities where mouldiness is easily developed, under the closed doors of cellars, you must certainly have more than once observed a tiny creature of the form of a horse-bean, of a gray leaden colour, and supplied with a considerable number of feet. This last characteristic will induce you immediately to abandon the idea that you have before you an insect.
Catch hold of it, and count its feet.
Well said; but it runs much more quickly than I would have suspected from its previously dilatory movements.
Because it knows that danger threatens it, instinct impels it to escape at its utmost speed. Do not be afraid to handle it; the poor creature can do you no harm.
Unable to escape, it counterfeits death, and remains perfectly immovable.
Now examine it. It has fourteen feet, symmetrically arranged in couples; their size perceptibly increases from the first to the last. When the animal is at rest, they are coiled inside, so as to form an angle whose opening faces the medial line. But here is something much more curious; its body, which does not possess the vestige of a wing, is also without those segments which would divide it visibly, as in the case of insects, into head, thorax, and abdomen; but is composed of rings, hard and scaly, like those of a shrimp.
Can it be a crustacean?
Yes; the animal you hold in your hand, and which everybody knew by the name of wood-louse long before our naturalists knew how to classify it, belongs to the great animal division of the Articulata, which, instead of having their skeleton inside the body, like the Vertebrates, have it externally. The Crustacea form a class of this division, to which also belong the Insects, the Arachnida, and the Myriapoda.
Let us continue to anatomise our crustacean.