Which structure is absent in invertebrates?

Invertebrates are a diverse group of animals that, by definition, lack a backbone or vertebral column. Among the various structures found in animals, a skeleton, as understood in the context of vertebrates, is often absent in invertebrates.

Invertebrates can be classified into several categories, including arthropods, mollusks, annelids, cnidarians, and others. While some invertebrates may have a form of structural support (like an exoskeleton in arthropods or a hydrostatic skeleton in soft-bodied invertebrates like jellyfish), they do not possess an internal skeleton made of bone like vertebrates do.

The concept of a skeleton in vertebrates often refers to the complex bones that provide support and protect the organism's internal organs, which are not present in invertebrates. In summary, while invertebrates may have alternative structures for support and movement, they lack the typical bony skeleton characteristic of vertebrates, making the absence of a skeleton in its more traditional sense a definitive trait of invertebrates.