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Why is the category invertebrates difficult to treat as one natural group?

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A diagram showing the hierarchy of animal classification in taxonomy, with colored boxes arranged from broadest to most specific: domain, kingdom, phylum, class, order, family, genus, and species. — taxonomy Animals and other organisms are classified within a succession of nested groups that ranges from the general to the particular.

In biological taxonomy, the term "invertebrate" is not considered a natural group because it is a paraphyletic grade rather than a true clade.1

A natural group, or clade, must include a common ancestor and all of its descendants. In contrast, "invertebrate" is defined solely by the absence of a vertebral column, a negative characteristic that encompasses a vast, diverse array of animals distributed across more than 30 different phyla. Because this category excludes vertebrates—which are themselves a specific lineage within the phylum Chordata—it fails to represent a single, unified evolutionary lineage.12

Essentially, "invertebrate" serves as an informal, catch-all label for all animals that are not vertebrates. Beyond the lack of a backbone, these organisms share very little in common, ranging from microscopic organisms to complex creatures like squid and insects. Consequently, there is no formal taxonomic group recognized as "Invertebrata" in modern systematic biology.12