Cephalochordata may be the most primitive subphylum of chordates and the sister group of the vertebrates. However, recent molecular studies (Benton 2005) place cephalochordates near the vertebrates and most authors also regard Amphioxus as the closest relative of Vertebrates.
With about 25 species inhabiting shallow tropical and temperate oceans, Cephalochordata are a small branch of animal kingdom, which includes lancelets or Amphioxus that spend much of their time buried in sand. Lancelets can swim vigorously forward and backward but they spend most of their time buried halfway in the sand. They live in masses of more than 9000 animals per square meter. In coarse sand the entire body is buried, with only the head exposed to water.
Lancelets are whitish to creamy yellow in colour, sometimes with a tint of pink. They have 56-64 muscle blocks and reach a length of 6 centimetres. The notochord extends along the entire length of their body and permits lateral swimming movements. The segmental arrangement of the myonemes facilitates the lateral movements during swimming.
A long and narrow dorsal fin, a posterior caudal fin and the short ventral fin are supported by fibrous fin rays to increase their effectiveness during locomotion. The ventral fin runs between anus and atriopore. In front of the atriopore paired fin-like folds, called the metapleural folds, extend to the anterior region of the pharynx.
The pharynx is perforated by over 100-150 pharyngeal gill slits or stigmata, which are used to strain food particles out of water. In cephalochordates a vestibule lies in front of the pharynx that is guarded by the oral cirri. The vestibule contains ciliated tracts of wheeler organ, an excretory organ (Hatschek’s nephridium), and a shallow groove (Hatschek’s pit) that is probably homologous to the adenohypophysis in vertebrates.
Along its ventral midline runs a groove, called the endostyle that produces mucus and thyroxine. The gill slits do not open to the outside directly but the whole pharynx is enclosed in a chamber called the atrium. The atrium opens in the ventral midline through an atriopore anterior to the anus.
Water is taken in through the mouth which is guarded by oral cirri, drawn in by the beating of cilia located on the wheeler organ or Muller organ, a set of ridges lying inside the mouth. It then passes through the pharyngeal gill slits that are enclosed in a form of body cavity known as the atrium.
Food particles in the water are trapped by mucus, while water passes through the slits and out of the atrium through the atriopore, located towards the posterior end. The food particles together with mucus are rolled into a mass that is passed on into the oesophagus and the posterior intestine where they are digested. The digestive system includes a hepatic caecum that secretes digestive enzymes for digestion of food in it and in the intestine.
Cephalochordates also have a well-developed circulatory system that resembles vertebrate system but does not have a heart but a simple sinus venosus. Circulation of blood in dorsal and ventral aortae is also like in vertebrates.
The dorsal nerve cord of cephalochordates has a central canal that is enlarged at the anterior end forming cerebral vesicle that has infundibular organ, Reissner’s fibers and cerebral eye. Also there are numerous pigmented ocelli throughout the length which function as photoreceptors.
In addition, several more structures in the neural cord are probably light sensitive, e.g., the Joseph cell receptors and the lamellar body in the cerebral vesicle. A photoreceptor in the anterior cerebral vesicle of larvae shows significant similarities to the paired eyes of vertebrates and could be homologous to them. On the skin there are receptors called corpuscules de Quatrefage, which are thigmoreceptors concentrated on the buccal cirri, velar tentacles, the atrium and along the metapleural folds.
The excretory organs of cephalochordates are unique in the animal kingdom, probably derived from the so-called podocytes that are found in other deuterostomes. They are called protonephridia found serially along the dorsal part of the pharynx and associated with blood vessels, and function in a similar way as the vertebrate kidneys. In addition to these, a nephridium of Hatschek is situated dorsally just in front of the pharynx.
Sexes are separate and both males and females have 26 pairs gonads. Eggs are fertilized externally in water and develop into free-swimming, fish-like larvae. Eggs are about 100 µm in diameter and contain numerous yolk droplets. Holoblastic and radial cleavage is followed by the formation of a blastula and then gastrula. An asymmetrical larva hatches from the egg, which has large mouth situated on the left side and a single row of gill slits that open to the outside.
More gill slits are added later an additional row of gill slits forms on the right side. When about 12–15 pairs of gill slits are formed the larva sinks to the bottom and metamorphoses into adult. The larval period in the plankton stage lasts from several weeks to a few months. In the following year the animals become sexually mature.
Pikaia gracilens from the Canadian Burgess Shale (Cambrian, 530 million years old) is considered to be a cephalochordate fossil. It has a body tapered at both ends and with a notochord on the posterior two-third. However, Pikaia also shows characters not found in extant cephalochordates, e.g. myotomes are W-shaped rather than of V-shape of cephalochordates.
Also, the anterior end displays tentacles, a feature not found in living cephalochordates. Several other fossils, e.g., Cathaymyrusdiadexus [Cambrian], Lagenocystispyramidalis [Ordovician] and Palaeobranchiostomahamatotergum [Permian]) also appear to fall in Cephalochordata. The fossil of Yunnanozoonlividum (Cambrian), earlier believed to be a cephalochordate, was reclassified as a hemichordate and later interpreted as cnidarian.