Select the vertebrate.

 - opossum
 - bess beetle
 - rock lobster
 - fireworm
A bess beetle is an insect. Like other insects, a bess beetle is an invertebrate. It does not have a backbone. It has an exoskeleton.
A rock lobster is a crustacean. Like other crustaceans, a rock lobster is an invertebrate. It does not have a backbone. It has an exoskeleton.
An opossum is a mammal. Like other mammals, an opossum is a vertebrate. It has a backbone.
A fireworm is a worm. Like other worms, a fireworm is an invertebrate. It does not have a backbone. It has a soft body.