testudo

The drawing you have already seen. The retiriĝi page at rank 2,352 is illustrated with a tortoise pulling its head into its shell, because there is no better picture anywhere of what withdrawing means. That art was made before this word had a page. Now the animal has one of its own, and the two drawings share a shell on purpose: the same dome, the same segment lines, the same plinth of a body underneath.

If you learn the two together, each one holds the other up. Retiriĝi is what a testudo does when you get too close.

One root, and the -ud- is not a suffix. Esperanto has -uj-, -ul-, -um- and -ing-. It has no -ud-. Testud- is a single borrowed root and test- is not hiding inside it: the Latin testa behind it is a pot, not a trial, and the English test that shares the root got there through the potter's crucible rather than the exam hall. Do not split it.

One word, two animals. English separates the land tortoise from the sea turtle. Esperanto does not, and neither do French (tortue) or Spanish (tortuga). When it matters you specify: martestudo is a sea turtle, tertestudo a land tortoise, akvotestudo a freshwater terrapin. Most of the time it does not matter, and testudo on its own is the animal.

The shell. The precise word is karapaco, the technical term for the domed upper shell. In ordinary prose people say ŝelo or testudŝelo, and everybody understands. The Roman soldiers who locked their shields into a roof called the formation a testudo too, which is a compliment to the animal.

The proverb travels. The hare and the tortoise is la leporo kaj la testudo, and the moral survives the crossing intact: malrapide sed certe.