Select the most appropriate one-word substitution for the given group of words.

The class of animals without backbone 

Invertebrates

The most appropriate one-word substitution for "the class of animals without backbone" is invertebrates.

• Spineless can refer to any animal that does not have a backbone, but it is not a very specific term.
• Biped refers to an animal that walks on two legs.
• Reptiles are a class of animals that includes snakes, lizards, turtles, and crocodiles.
• Invertebrates is a class of animals that does not have a backbone. It includes over 90% of all animal species on Earth.