Chordate of the three subdivisions of chordates, all have distinct skulls. They include the hagfish, which have no vertebrae. Michael J. Benton commented that "craniates are characterized by their heads, just as chordates, or possibly all deuterostomes, are by their tails". Most craniates are vertebrates, in which the notochord is replaced by the vertebral column. These consist of a series of bony or cartilaginous cylindrical vertebrae, generally with neural arches that protect the spinal cord, and with projections that link the vertebrae. However hagfish have incomplete braincases and no vertebrae, and are therefore not regarded as vertebrates, but as members of
Progression of Animals Progression of Animals Progression of Animals (or On the Gait of Animals; ; ) is one of Aristotle's major texts on biology. It gives details of gait and movement in various kinds of animals, as well as speculating over the structural homologies among living things. Aristotle's approach to the subject is to ask "why some animals are footless, others bipeds, others quadrupeds, others polypods, and why all have an even number of feet, if they have feet at all; why in fine the points on which progression depends are even in number." It is a good example of the way