: amphioxus /ˌæmfiˈɒksəs/ AM-fee-OK-səs), consist of 32 described species of "fish-like" benthic filter feeding chordates[9] in the subphylum Cephalochordata, class Leptocardii, and family Branchiostomatidae.

The circulatory system carries food throughout their body, but does not have red blood cells or hemoglobin for transporting oxygen.

In Japan, amphioxus (B. belcheri) has been listed in the registry of "Endangered Animals of Japanese Marine and Fresh Water Organisms".

[20] Adult amphioxus typically inhabit the seafloor, burrowing into well-ventilated substrates characterized by a soft texture and minimal organic content.

All amphioxus species exhibit gonochorism, with only rare instances of hermaphroditism reported in Branchiostoma lanceolatum and B. belcheri.

An extraordinary occurrence of complete sex reversal was documented in B. belcheri, where a female amphioxus raised in laboratory conditions underwent a transformation into a male (Zhang et al., 2001).

Nicholas and Linda Holland were the first researchers to describe a method of obtaining amphioxus embryos by induction of spawning in captivity and in vitro fertilization.

[28] It was not until 1834 that Gabriel Costa brought the phylogenetic position of the group closer to the agnathan vertebrates (hagfish and lampreys), including it in the new genus Branchiostoma (from the Greek, branchio = "gills", stoma = "mouth").

[35] Alexander Kowalevsky first described the key anatomical features of the adult amphioxus (hollow dorsal nerve tube, endostyle, segmented body, postanal tail).

[39] Other important contributions to amphioxus embryonic anatomy were given by Hatschek, Conklin[40] and later by Tung (experimental embryology).

[47] Except for the size, the species are very similar in general appearance, differing mainly in the number of myotomes and the pigmentation of their larvae.

While they do possess some cartilage material stiffening the gill slits, mouth, and tail, they have no true complex skeleton.

[50] Unlike vertebrates, the dorsal nerve cord is not protected by bone but by a simpler notochord made up of a cylinder of cells that are closely packed in collagen fibers to form a toughened rod.

However, developmental gene expression and transmission electron microscopy indicate the presence of a diencephalic forebrain, a possible midbrain, and a hindbrain.

[54] Lancelets have four known kinds of light-sensing structures: Joseph cells, Hesse organs, an unpaired anterior eye and lamellar body, all of which utilize opsins as light receptors.

[60] Lancelets naturally express green fluorescent proteins (GFP) inside their oral tentacles and near the eye spot.

[citation needed] It is suspected GFP plays multiple roles with lancelets such as attracting plankton towards their mouth.

Considering that lancelets are filter feeders, the natural current would draw nearby plankton into the digestive tract.

Lancelets are passive filter feeders,[14] spending most of the time half-buried in sand with only their frontal part protruding.

[66] They eat a wide variety of small planktonic organisms, such as bacteria, fungi, diatoms, and zooplankton, and they will also take detritus.

[67] Lancelets have oral cirri, thin tentacle-like strands that hang in front of the mouth and act as sensory devices and as a filter for the water passing into the body.

The ventral surface of the pharynx contains a groove called the endostyle, which, connected to a structure known as Hatschek's pit, produces a film of mucus.

Ciliary action pushes the mucus in a film over the surface of the gill slits, trapping suspended food particles as it does so.

Having passed through the gill slits, the water enters an atrium surrounding the pharynx, then exits the body via the atriopore.

[49] Both adults and larvae exhibit a "cough" reflex to clear the mouth or throat of debris or items too large to swallow.

By the mid-20th century they had fallen out of favor for a variety of reasons, including a decline of comparative anatomy and embryology, and due to the belief that lancelets were more derived than they appeared, e.g., the profound asymmetry in the larval stage.

[citation needed] When their gonads start to ripen in the spring it affects their flavor, making them taste bad during their breeding season.

Extensive molecular phylogenetic analysis has shown convincingly that the Cephalochordata is the most basal subphylum of the chordates, with tunicates being the sister group of the vertebrates.

[67] The cladogram presented here illustrates the phylogeny (family tree) of lancelets, and follows a simplified version of the relationships found by Igawa, T.; M. Nozawa; D.G.