Echinodermata
The phylum Echinodermata takes its name from the Greek ekhinos for hedgehog and derma meaning skin, referring to the spiny skin of many species. The earliest known echinoderm fossils date back to the early Cambrian, 580 million years ago.
Echinoderms, one of the BIG 9 phyla, are the only animals with radial, often five-sided, symmetry. They are in the deuterostome group of animals along with humans. Echinoderms start life as bilateral free-swimming larvae, developing the characteristic radial symmetry as adults. Echinoderms have skeletons containing pieces of calcite (calcium carbonate crystals) called ossicles covered by skin. In some echinoderms, including sand dollars and sea urchins, the ossicles form a rigid shell known as a test as well as spines.
Chordata
Chordates are united by a structure known as the notochord. This stiff, rod-like structure lies below the nerve cord, providing a rigid base against which muscles can contract. If the notochord had not evolved, there would be no elephants, sharks, frogs, chickens, snakes or humans.
Chordates also have a hollow nerve cord running along their backs, a post-anal tail and pharyngeal gill slits at some point in their development.
Agnatha
The first fish appear in the fossil record more than 500 million years ago. These were jawless fish (Agnatha or Cyclostomata), covered in bony plates. The only agnathans to survive today are the hagfish and lampreys which diverged early in vertebrate evolution, before jaws and paired limbs appeared. Both are slippery, eel-like fish with cartilage skeletons.
Chondrichthyes
Chondrichthyes, the sharks, chimaeras, skates and rays, first appeared during the Devonian period around 400 million years ago. Chondrichthyes have unusual scales called dermal denticles which are structurally similar to teeth. Each scale has a hollow cone of dentine around a pulp cavity covered by an enamel-like substance. These tooth-like scales are very rough to the touch, like sandpaper.