Select the invertebrate.

 - saturn butterfly
 - robin
 - Galapagos giant tortoise
 - ground squirrel
A saturn butterfly is an insect. Like other insects, a saturn butterfly is an invertebrate. It does not have a backbone. It has an exoskeleton.
A ground squirrel is a mammal. Like other mammals, a ground squirrel is a vertebrate. It has a backbone.
A Galapagos giant tortoise is a reptile. Like other reptiles, a Galapagos giant tortoise is a vertebrate. It has a backbone.
A robin is a bird. Like other birds, a robin is a vertebrate. It has a backbone.