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
Non-reproductive sexual behavior in animals have provided good evidence that animals have instinct and a reward system. The behavior of laboratory animals demonstrates a mental experience wherein the animal's instincts tell it if it carries out a certain action, it will then receive what it needs. For example, the lab rat will push the lever because it knows food will fall out of the hole in the wall. It does not need awareness, but it does seem to work on a reward system. The lab rat learned the action needed to be fed. Studies of the brain have proven that pleasure and displeasure are an