Amphioxus – Characteristic, Functions and Their Habitat

Amphioxus is the goal of this guide, animals which are included in the Cephalochordata group, inside the Chordata phyllum. Cephalochordata is a group of marine animals put between invertebrates and vertebrates. And today we are going to explain the value of the animals in Zoology and its own physics.


Amphioxus, placed in the Cephalochordata subphyllum, is a marine creature in the Chordata group. Chordata contains, in addition to the group, Urochordata (one of which there is Pyrosomida), hagfishes and vertebrates (fishes, amphibians, reptiles, mammals and birds). Despite they represent only a 4% of the amount of organisms in the planet (that correspond to 55,000 species), Chordata has had a crucial evolutionary success.

The importance in Zoology of amphioxus is that present all the features of Chordata visible, so additional chordata has lost them later or has modified them. These are the attributes:

  • Notochord: dorsal bar placed under the nervous system with a skeletal purpose.
  • Epineuria: dorsal place of neural cord.
  • Endostyle: ventral groove at the pharynx that produce mucus to catch food and also create iodized compounds. This provides thyroid.
  • Caudal fin: locomotive appendix.
Basic features of Chordata
Basic features of Chordata in a Cephalochordata



Cephalochordata, called amphioxus, is a set of 25 species of marine creatures with a thin body, laterally compressed and transparent, that measures between 5 and 7 cm.

Brachiostoma lanceolatum


The skin of cephalochordata is determined by one layer of prismatic cells with mucus glands that produce mucus, followed closely by the rectal connective lamina and the dermis. The most feature is notochord, which consists of cells surrounded by a conjunctive instance of actin and paramyosin. These cells have neurons that come from the nerve pathways, allowing their contraction in diameter.

General anatomy of a cephalochordate
General anatomy of a cephalochordate 1. brain-like blister 2. notochord 3. dorsal nerve cord 4. post-anal tail 5. anus 6. food canal 7. blood system 8. abdominal porus 9. overpharynx lacuna 10. gill’s slit 11. pharynx 12. mouth lacuna 13. mimosa 14. mouth gap 15. gonads (ovary/testicle) 16. light sensor 17. nerves 18. abdominal ply 19. hepatic caecum 20. swim bladder 21. lateral line

They’re swimming creatures, with various fins: they got a dorsal fin, with vesicles placed one after another; a caudal fin and an anal fin, which extends from caudal fin until atriopore, opening from where water leaves the body. This anal fin bifurcates in two sheets and give place two folds to slightly stabilize them, which are called metapleural folds. They have a string of muscular fascicles called myomeres, which are in a shape of V with the apex in a forward position.

Oral region has an oral hood cirri to differentiate the entering particles, the Wheel organ (produce water movements) and a diaphragm to regulate the water entrance into your system. Pharynx is perforated for 80 fissures humor the endostyle in the foundation, that produce mucus and it is pick to a dorsal lamina, in which you will find a small pubs and then goes to oesophagus.


In order to feed, water with particles becoming in through the mouth, it is propelled by the oral hood cirri and then cross the gill’s fissures, where food gets stuck thanks to mucus produced by endostyle, and finally goes to intestines. Here, food particles proceed to an hepatic cecum and phagocytosis process occurs. Then, water travels to the inner cavity of the human body (known as atrium) and leaves the body through a pore (atriopore). Digestive system is compiled by the oral system, the pharynx with endostyle, the oesophagus and also a digestive tract without muscles; that can be composed in precisely the same time by the intestine, the bronchial cecum (produce enzymes and absorb nutrients) and the anus, placed in the left side of their human body. Its motion is a result of a cilium ring.

Circulatory system does not have heart and includes on two circuits: the ventral circuit extends from caudal fin to go and the dorsal, the other way round. The circulatory liquid belongs to pharynx fissures to become oxygenated and contains amebocytes, but it’s not respiratory pigments, therefore breathing takes places by diffusion. Excretory system is formed by solenocytes, cells that filter the blood from arteries, placed from the nefritic crest, that joins the atrium with a channel, so that allows that excretory products are expelled with the water from the atrium.

Nervous system is composed on a simple nerve cord with a vesicle in the anterior part. This cord, in each metamere, emits two dorsal mixed nerves (with motor and sensitive nerves), which can be branched off in two branches: a more sensitive dorsal branch along with a combined ventral branch. This ventral division goes to viscera, tegument and muscles. Sensitive system is comprised by a pigment place (sensitive to light) and chemoreceptors.

About reproduction, each creature has only 1 sex (dioic animals), but its own anatomy is very similar. They pose between 25 and 38 gonads and also to perform the lay, the body wall is broken.


Amphioxus resides buried in sand seafloor of the coastal and shallow waters and in estuaries all around the world.

Branchiostoma lanceolatum
Common amphioxus

Notes of the Chordata subject of the Degree in Biology of the University of Barcelona
Brusca & Brusca (2005). Invertebrates. Ed. Mc Graw Hill (2 ed)
Hickman, Roberts, Larson, l’Anson & Eisenhour (2006). Integrated principles of Zoology. Ed. Mc Graw Hill (13 ed)
Cover picture: Ricardo R. Fernandez