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
Really Wild Animals it home (Bird's-Eye View); and the fourth is about the importance of keeping the rainforest safe (This Must Be the Place). This episode takes a look at the North American continent from its deserts and swamps to its cities and tundra. Ground squirrels, polar bears, alligators, beavers, woodpeckers, Arctic wolves, rattlesnakes, musk oxen, raccoons, black bears, corn snakes and red-bellied turtles are all featured. The diversity of the North American landscape, and how all its animals have adapted to it, are all key points in the film. This episode contains four songs. The first one is about what animals need