Cephalochordata are primitive, acraniate (without a skull) fish-like chordates. They have no scales; nor do they have bones and cartilage as true fish have. Cephalochordates posses a notochord, a dorsal hollow nerve cord, pharyngeal gill slits, and a myotomal musculature. The notochord gives stiffness to the body.
The adult head bears twelve oral tentacles (or cirri), with which the animal selects larger particles from the water. Cilia in the pharynx generate a water current through the gill slits, from where the water flows into the atrium, and then to the exterior by the atriopore. An endostyle secretes a mucus sheet covering the gill slits, trapping small food particles. The gill slits functioning thus in both feeding and gas exchange. There is a hepatic caecum, and there are two nephridia for excretion.
Cephalochordata are of separated sexes. The gonads are ventrally from the frontal part of the intestine. The fertilisation of the eggs is external; the eggs hatch as ciliated neurulae, further developing into free-swimming larvae that resemble the "tadpole" larvae of ascidian tunicates.
Cephalochordata are cosmopolitan in shallow marine and brackish waters from tropical and temperate regions; some 28 species are described. In the North Sea only one cephalochordate species occurs, Branchiostoma lanceolatum.