The Vertebrates
Vertebral column replaces notochordduring embryology (except earliest fish); vertebrae are made of cartilage or bone and enclose and protect the dorsal nerve cord
Head, in which the brain is encased in a skull made of bone or cartilage (craniate chordates)
Bones are made of compacted fibers of the protein collagen (flexibility) encrusted with calcium phosphate minerals (rigidity) ? bone (and cartilage) are tissues
Internal organs characteristic for vertebrates: liver, kidneys, endocrine glands, heart with closed blood system