How do ball-and-socket joints in vertebrates compare with those in millipedes and sea urchins?

In vertebrate anatomy, a ball-and-socket joint consists of a rounded bone surface rotating within a concavity, such as the acetabulum, allowing for movement in multiple planes. While these joints are highly developed in the shoulder and hip of mammals, they are also found in the vertebrae of various vertebrates, including fishes, amphibians, reptiles, and birds.12

Invertebrates exhibit remarkable parallels to these structures. In certain millipedes, ball-and-socket-like mechanisms allow for the rotation and flexion of heavy, circular skeletal rings, enabling the animal to curve its body in any direction. Similarly, sea urchins possess numerous ball-and-socket joints located at the base of their movable spines.2

While vertebrates typically utilize these joints within an endoskeleton to facilitate complex limb movement, the examples in millipedes and sea urchins demonstrate how similar mechanical principles—a ball-like facet rotating within a socket—have evolved independently in invertebrates to serve different functional needs, such as body flexibility in millipedes and spine articulation in echinoderms.2