The lancelet, fish external structure and Lab 4

Chordates have an axial skeleton, the notochord, running along the body; the neural tube lies above it and the gut below; they breathe with gills or lungs. The phylum divides into the subphyla Acrania and Craniata (vertebrates). The lancelet (4–8 cm) is an acranian: it lives buried in sand on the sea floor, its body is transparent and flattened from the sides, and its mouth is ringed with tentacles. The notochord stays for life, and there is no brain or skull. It feeds on plankton: tentacles drive a water flow into the mouth, food sticks in the pharynx and passes to the gut, and water leaves through gill slits. Circulation is closed, a single circuit, with no heart. The nervous system is a neural tube; sense organs are weak. Sexes are separate and eggs are fertilised in water. The lancelet is important for understanding chordate origin: its notochord, neural tube and gill slits link it to chordates, while its excretory organs resemble those of invertebrates. In craniates the notochord exists only in the embryo, and the central nervous system consists of brain and spinal cord.