Select the invertebrate.

 - birdwing butterfly
 - dwarf crocodile
 - rainbow trout
 - yak
A yak is a mammal. Like other mammals, a yak is a vertebrate. It has a backbone.
A birdwing butterfly is an insect. Like other insects, a birdwing butterfly is an invertebrate. It does not have a backbone. It has an exoskeleton.
A rainbow trout is a fish. Like other fish, a rainbow trout is a vertebrate. It has a backbone.
A dwarf crocodile is a reptile. Like other reptiles, a dwarf crocodile is a vertebrate. It has a backbone.