See PJH p. 195 for a more realistic interpretation of the jelly roll than the one I showed in class. Our first part course is about connections between vertebrates. Simply put, vertebrates are connected to one another by their similarities. In other words, they share characters with one another. Their defining characters are called shared-derived characters, and we will deal with this meaty concept in depth during the term, beginning in the next lecture.
Bone is an example of a shared vertebrate character.
Many, but not all vertebrates have bone. The whale vertebra self test illustrates that many vertebrates share in having a segmented 'backbone', or vertebral column. Many, but not all vertebrates make a vertebral column. A notable one that does not is the hagfish . Possibly the ancestors of the hagfish had not ever invented this structure.