Now consider the turtle again.

The turtle. A hard shell outside, jointed legs, and a backbone inside.

It has a hard shell on the outside, and it has jointed legs.

But inside, the turtle has a backbone. That is one no.

The turtle is not an arthropod.

The turtle: a cross under No backbone? Not an arthropod.

Now consider a snail.

A snail. A hard shell, a soft body, and no legs.

It has no backbone, and its shell is a hard outer casing.

But it has no legs at all. That is one no.

The snail is not an arthropod.

The whole table. Three arthropods; the turtle and the snail each have one no.

An arthropod has all three features. One no is enough to keep an animal out.