Invertebrates
Lancelet
Branchiostoma · also called lancelets, amphioxus
A lancelet lies mostly buried in shallow sand, only its front end poking into the water to filter feed, an animal so simple it has no brain, no jaws, and no backbone. Yet running the length of its thin, translucent body is a stiff supporting rod and a nerve cord above it, the same basic blueprint every vertebrate embryo briefly carries before growing something more complicated.
Overview
The lancelet, genus Branchiostoma, is a small, blade-shaped filter feeder found buried in sandy seafloor habitat in coastal waters worldwide. It lacks a brain, jaws, and a true backbone, retaining instead a notochord and simple nerve cord.
A living blueprint
Because its notochord, nerve cord, and segmented muscle blocks mirror the earliest stage of every vertebrate embryo, the lancelet has served researchers for well over a century as a working model for understanding how the vertebrate body plan first came together.
Status
Least Concern — common and widespread across suitable sandy coastal seafloor habitat worldwide.
Lancelet: common questions
Why is the lancelet considered so important to understanding evolution?
A lancelet has a stiff notochord, a hollow dorsal nerve cord, and segmented muscle blocks, the same basic set of features found in the earliest embryonic stage of every vertebrate, and because the lancelet retains these features into adulthood without ever developing a backbone, it serves as a genuinely useful living reference point for understanding what the ancestors of vertebrates may have looked like.
Does a lancelet have a brain?
No, a lancelet does not have a true brain, relying instead on a simple hollow nerve cord running along its back, and it also lacks jaws and paired eyes, retaining a body plan considered more primitive than that of any true vertebrate while still sharing several of the same fundamental structural building blocks.
Where does a lancelet spend most of its time?
A lancelet typically spends much of its life partly buried in sandy or gravelly sediment on the shallow seafloor, with only its front end protruding into the water column to filter feed, and it can burrow back into the sediment quickly if disturbed by a potential predator.
How does a lancelet actually feed?
A lancelet draws water in through its mouth and passes it through a mucus-lined structure inside its pharynx that traps plankton, algae, and other fine organic particles as the water flows through, a filter-feeding method broadly similar to that used by several other simple marine invertebrates.