What does the endoskeleton consist of in vertebrates?

The endoskeleton of vertebrates primarily consists of an internal framework made of bones and cartilage, which provides structural support, protects vital organs, and allows for mobility due to the attachment of muscles. This bony and cartilaginous structure is crucial for the overall functioning and movement in vertebrate animals.

Bones provide hard support and protection for internal organs, while cartilage offers flexibility in areas such as joints and the respiratory system. This combination ensures that vertebrates can maintain their shape, move efficiently, and protect delicate inner structures.

In contrast, the other choices do not accurately define the endoskeleton. Muscles and skin, while integral to vertebrate anatomy, do not form the skeleton. A network of blood vessels is crucial for transportation within the body but does not constitute a skeletal system. Finally, while connective tissue is a part of the endoskeleton, describing it solely as made of connective tissue overlooks the specific roles and properties of bones and cartilage that make up the endoskeleton. Therefore, the correct characterization is that the endoskeleton is indeed an internal system of bones and cartilage.