A group of living things
Chordates
Chordata
Animals with a stiff rod down the back at some stage: the blueprint of a backbone.
Its members' last common ancestor lived about 550 million years ago.
You are part of this group. See how.
Every chordate carries a stiff, bendy rod down the back at some stage of life. In lancelets and tadpole larvae, muscles pull against it to swim. The rod comes with three companions: slits linking the throat to the outside, a hollow nerve cord above the rod, and a tail behind the rear opening of the gut. That blueprint links animals that look nothing alike. Sea squirts and vertebrates sit together inside this group, while starfish sit just outside as close relatives.
Adult sea squirts are anchored sacs or barrels with two openings. They draw water in one opening to collect food and oxygen, then squirt it out the other. Lancelets are small, see-through animals that keep the full set as adults but never grow a backbone, proper brain or true eyes. Almost all vertebrates add a skull and backbone, usually with two eyes and paired fins, four limbs or wings.
Many sea squirts start as free-swimming tadpoles with the same four features, then settle, lose the tail, and much of the nervous system breaks down. Apart from vertebrates, chordates live only in the sea, while vertebrates live from oceans and fresh waters to land and air. Sizes run from tiny tadpole-like drifters about as long as a grain of rice to blue whales about as long as three buses.
Coats of plant fibre
Sea squirts and their kin are the only animals that make cellulose. They use it to build a tough, leathery outer coat, with a gene once picked up from bacteria.
Two sexes, plus budding
Most sea squirts carry both male and female organs but cannot fertilise themselves and need a neighbour to breed. Kinds that live in colonies can also bud off new bodies without mating.
Crowded sands eaten as food
Lancelets can crowd shallow sand at about 5,000 animals in each square metre. In parts of Asia people harvest them for food.