Answer:
c. They have a cranium and a notochord composed of cartilage
Explanation:
The presence of the vertebral column is the key feature of vertebrates. Cranium and vertebral column constitute the endoskeleton of these organisms. Hagfishes are the jawless fishes that have the skeleton (cranium and notochord) made up of cartilage, rather than bones as it is in other vertebrates. Hagfishes are classified under the class Myxini and are characterized by the presence of notochord as the only axial structure to support the body.
Since hagfishes lack vertebrae, they are sometimes classified as craniates, not as vertebrates. The cartilaginous notochord persists throughout life. It is not replaced by the bony vertebral column. The cartilaginous segments that represent the rudiments of vertebrae are called neural arches.
