What is a defining feature of chordates?

A defining feature of chordates is the possession of an internal skeleton made of bone and cartilage. This structure provides support and allows for a range of movement, enabling more complex body forms and functions compared to organisms with exoskeletons or external supportive structures. The internal skeleton can grow with the organism and is a key component in the development of sophisticated systems, such as muscles and organs.

While exoskeletons and other external structures are found in different animal groups—like arthropods with their chitin-based exoskeleton—chordates are unique in their internal supportive framework. The internal skeleton also contributes to the protection of vital organs and provides a point for muscle attachment, which is crucial for mobility and other functions. This characteristic is one of the many features that separates chordates from other groups in the animal kingdom.