Select the invertebrate.

 - sea turtle
 - red-spotted purple butterfly
 - koala
 - puffin
A red-spotted purple butterfly is an insect. Like other insects, a red-spotted purple butterfly is an invertebrate. It does not have a backbone. It has an exoskeleton.
A puffin is a bird. Like other birds, a puffin is a vertebrate. It has a backbone.
A koala is a mammal. Like other mammals, a koala is a vertebrate. It has a backbone.
A sea turtle is a reptile. Like other reptiles, a sea turtle is a vertebrate. It has a backbone.