What does amphioxi mean?
Amphioxi are the plural form of amphioxus, the name given to small, slender, fish-like marine animals more commonly called lancelets. Belonging to the subphylum Cephalochordata, they inhabit shallow coastal waters worldwide, burrowing in sandy sea floors and filter-feeding with their heads protruding. Though superficially fish-like, amphioxi lack a true backbone, jaws, eyes, and paired fins; instead they possess a flexible rod-like notochord running the length of the body — a defining chordate feature. This simplicity makes them invaluable to evolutionary biology, since they represent one of the closest living relatives of vertebrates and offer a glimpse into the ancestral body plan from which fishes, and ultimately humans, emerged. The word derives from Greek roots meaning 'sharp at both ends,' describing the animal's pointed shape. Amphioxi appear almost exclusively in scientific contexts, where the singular amphioxus or the genus name Branchiostoma is more common.
nounAmphioxi are small, fish-like marine animals of the genus Amphioxus (also called Branchiostoma), commonly known as lancelets; the term is the plural form referring to these chordates collectively.
- Small, fish-like marine chordates of the genus Amphioxus (lancelets), used as a plural collective term in zoology.
"The marine biologists dredged up dozens of amphioxi from the sandy seabed off Naples."
"Zoologists study amphioxi to understand the evolutionary origins of the vertebrate body plan."
"In his comparative anatomy lectures, Professor Hale often began with amphioxi as the simplest living chordate."
This is already the plural form; the singular is 'amphioxus'. In modern scientific usage the genus name Branchiostoma is often substituted.
"The tide pools revealed several amphioxi half-buried in the sand."
The amphioxus looks like a tiny translucent fish, but it has no backbone — making it one of our closest living invertebrate relatives and a living window into how all vertebrates evolved.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of amphioxi
Amphioxus was coined in the early 19th century from Greek elements: 'amphi' meaning 'on both sides' or 'around,' and 'oxys' meaning 'sharp' or 'pointed,' together describing the animal's tapering, pointed shape at both ends. The same Greek roots give English words such as amphibious and amphitheater (from 'amphi') and oxygen and oxide (from 'oxys'). The plural amphioxi follows the Latinized pattern for Greek nouns ending in -x.
Related word forms
How amphioxi is actually used
A technical zoological term, essentially confined to scientific writing. The singular 'amphioxus' is far more frequently encountered than the plural 'amphioxi'; in modern literature the generic name Branchiostoma is increasingly preferred.
Easily confused with amphioxi
Amphioxi are invertebrate chordates (lancelets) that live entirely in seawater, whereas amphibians are vertebrates such as frogs and salamanders that typically live both in water and on land.