Lancelets

Although they resemble eel-like fish, the humble lancelets are in fact a primitive group that are part way between those animals that do not have a backbone (invertebrates) and those that do (vertebrates).

Lancelets are small marine animals that spend much time burrowing in the seabed, although they can swim.Their narrow, fishlike body is pointed at both ends. They have a narrow dorsal (back) fin and a small, rounded tail fin.The head of a lancelet is small and has no eyes. There is a well-developed nerve cord that connects with a simple brain. The brain is not enclosed in a braincase.The mouth has no jaws or teeth, and

is surrounded by sensory tentacles. The mouth leads into the pharynx or throat. Unlike fish, whose gills are in direct contact with the water, the gills of lancelets are in a cavity connected to the pharynx.

Lancelets are filter feeders. They draw water into the gills through the mouth and sieve out food particles. The particles are collected and passed along a channel to the animal’s intestine. Lancelets use the gills to extract oxygen from the water at the same time.

How Backbones Evolved

The structure of a lancelet’s body tells us a lot about the phylum Chordata (the group that includes vertebrate animals such as humans) and their origins. Despite their fishlike appearance and behavior, lancelets have no backbone. Instead, a stiffening rod of fibers and jelly runs from the front of the animal to the rear. This supports the body and provides muscle anchorage. This structure is an evolutionary forerunner of the spinal column found in vertebrates.