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
Things Which Have Never Yet Been Done was shot with a handheld camera by the episode's director, TJ Scott, with Maslany and Outerbridge improvising the dialogue and use of props. The subsequent shot of the barn on fire was created by the visual effects company Intelligent Creatures. The title of the final two episodes, "Things Which Have Never Yet Been Done" and "By Means Which Have Never Yet Been Tried", are taken from a Francis Bacon quotation in his philosophical work "Novum Organum": "It would be an unsound fancy and self-contradictory to expect that things which have never yet been done can be done except by means