Use dichotomous keys to identify and classify organisms.

A student uses a dichotomous key to identify an unknown organism. At the first step, the key asks: 'Does the organism have a backbone?' The student observes that the organism has jointed legs, an external skeleton, and antennae. Which characteristic would the student use to answer this first branching question in the dichotomous key?

A student uses a dichotomous key to identify an unknown organism. At the first branch, the key asks: 'Does the organism have a backbone?' The student observes that the organism has no backbone, jointed legs, and a hard outer skeleton. Which of the following groups is the organism most likely to belong to?