Lancelet

The anatomy of a lancelet is well known and often used as a model when learning about the five chordate characteristics.

These characteristics are evident in an adult lancelet, and its body plan is considered to be ancestral for all chordates.

Lancelets swim using their segmented muscles called myomeres, which attach directly to the notochord.

Their movement is helped along by long flaps known as metapleural folds, but they lack the paired fins that are characteristic of the jawed vertebrates.

In order to feed, lancelets keep themselves partially buried in fine sediments, with only their anterior end exposed.

Water enters the mouth, driven by cilia in the buccal cavity and pharynx, and passes through the pharyngeal gill slits, where food is trapped in mucus secreted by the endostyle, and then moved into the gut, where food particles are separated from mucus and passed into the hepatic cecum, where they are digested.

Lancelets have a closed circulatory system, but lack a heart and pigmentation.

They also lack a respiratory system, and instead exchange gases directly through their skin.