What is one difference among animals that used to classify them into two large groups?

One of the primary criteria for classifying animals into two broad groups is the presence or absence of a backbone, dividing them into vertebrates and invertebrates.

Vertebrates:

Vertebrates possess a backbone or vertebral column, providing structural support and protection for the central nervous system. They also have specialized internal organs, organized systems, and a greater degree of complexity and mobility compared to invertebrates. Examples of vertebrates include mammals, birds, reptiles, amphibians, and fish.

Invertebrates:

Invertebrates lack a backbone and have a diverse array of body structures and characteristics. They represent the vast majority of animal species and inhabit a wide range of environments. Invertebrates are further classified into various groups based on their specific features and include insects, arachnids, mollusks, worms, and echinoderms, among others.

The distinction between vertebrates and invertebrates has historically played a significant role in categorizing animals, providing a fundamental division within the animal kingdom and forming the basis for further classification into smaller and more specific groups.