Temporal range: Cambrian Stage 3–Recent
|A Branchiostoma lanceolatum lancelet|
A cephalochordate (from Greek: κεφαλή kephalé, "head" and χορδή khordé, "chord") is an animal in the chordate subphylum, Cephalochordata. They are commonly called amphioxus or lancelets. They are chordates with all 5 synapomorphies, the characteristics all chordates have during the larval or adulthood stages. These synapomorphies include: notochord, dorsal hollow nerve cord, endostyle, pharynx and post-anal tail. The fine structure of the cephalochordate notochord is best known for the Bahamas lancelet, Asymmetron lucayanum  Cephalochordates are represented in modern oceans by the Amphioxiformes.
Characteristics of Cephalochordata include that they are segmented marine animals that possess elongated bodies with a notochord that extends the length of the body, extending from head to tail, persisting throughout the animal's life. The members of this subphylum are very small and have no hard parts, making their fossils difficult to find. Fossilized species have been found in very old rocks predating vertebrates. There is a famous fossil shale from the Middle Cambrian, the Burgess Shale of British Columbia, which has yielded Pikaia fossils. Recently, a different cephalochordate fossil (Yunnanozoon) has been found in south China. It dates to the early Cambrian period, and is the earliest known fossil of the cephalochordate lineage. Members of this lineage have numerous gill slits, and have separate sexes.
Cephalochordates are thought to have diverged from the rest of the chordate subphylum approximately 700 to 650 million years ago. Phylogeny is based on a combination of studies of extinct and extant species.
Cephalochordates employ a filter feeding system to consume microorganisms. The oral hood serves as the entrance for food particles, and possesses buccal cirri, which assist in sifting out larger food particles before they enter the buccal cavity. Epithelial cilia lining the mouth and pharynx form a specialized "wheel organ" situated at the dorsal and posterior end of the cavity. The motion of the cilia resembles the motion of a turning wheel, hence the organ's name, and transports the captured food particles. Behind this organ is the velum, which acts as an internal filter before food enters the pharynx. The food particles adhere to secreted mucus on the pharyngeal bars before being brought to the epibranchial groove on the dorsal side of the pharynx. Following this, the food is transferred to the gut, and excess water is pumped from the pharynx through the pharyngeal slits. This excess water passes through the atriopore and is then excreted from the body.
- Nielsen, C. (July 2012). "The authorship of higher chordate taxa". Zoologica Scripta. 41 (4): 435–436. doi:10.1111/j.1463-6409.2012.00536.x.
- Holland, Nicholas; Somorjai, Ildiko (2020). "Serial blockface SEM suggests that stem cells may participate in adult notochord growth in an invertebrate chordate, the Bahamas lancelet". EvoDevo. 11 (22).
- Erwin et al. (2011) The Cambrian Conundrum: Early Divergence and Later Ecological Success in the Early History of Animals. Science 334: 1091-1097.
- Haaramo, Mikko. Cephalochordata – lancelets. Retrieved 2013-10-22.
- Kon, T.; et al. (July 2006). "Hidden ancient diversification in the circumtropical lancelet Asymmetron lucayanum complex". Marine Biology. 149 (4): 875–883. doi:10.1007/s00227-006-0271-y. S2CID 85904664.
- V., Kardong, Kenneth (2015-01-01). Vertebrates : comparative anatomy, function, evolution. McGraw-Hill. ISBN 9780078023026. OCLC 910511685.
- Fishbeck, D. Sebastiani, A. (2015). Comparative Anatomy: manual of dissection. Morton Publisher Company.CS1 maint: multiple names: authors list (link)