What defines a vertebrate?

A vertebrate is defined as an animal that possesses a backbone or spinal column, which is a defining characteristic of this subphylum of the animal kingdom. The backbone is composed of individual vertebrae, forming a supportive structure that protects the spinal cord and allows for a more complex and flexible skeletal framework. This classification includes a wide range of animals, such as mammals, birds, reptiles, amphibians, and fish.

The other options present descriptions that do not align with the biological classification of vertebrates. The first option refers to animals that lack bone structures, which would include many invertebrates. The third option mentions sexual reproduction, which does not serve as a defining characteristic since many vertebrates also reproduce sexually. The fourth option describes invertebrates, specifically mentioning segmented bodies that are typical of organisms such as annelids, which do not fall within the vertebrate classification. Thus, the correct answer highlighting jointed backbones accurately reflects the key characteristic that defines vertebrates.