Tunicates spend their adult lives attached to a solid surface, and are closely related to vertebrates. In an early free-swimming larval stage tunicates resemble tadpoles with a nervous cord (notochord). A notochord is the primitive beginnings of a backbone and can be found in early developmental stages of human embryos. Once ready to become adults larvae will attach head first to a solid surface and begin their transformation. The adults are called tunicates because of the firm protective tunic that surrounds the animal. Adult tunicates are filter feeders and have two siphons – the oral siphon brings fresh seawater into the tunicate where plankton and oxygen can be retained, and the second atrial siphon releases waste and allows water flow through the animal.