How do cephalochordates move through water?

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A translucent X-ray fish (Pristella maxillaris) swims in an aquarium, showing its visible backbone and internal structures through its body. — Chordate X-ray fish (*Pristella maxillaris*), is an example of a chordate with a visible backbone.

Cephalochordates, also known as lancelets or amphioxi, are capable of swimming, though they spend much of their time buried in sand or gravel on the ocean floor.12

When swimming, they move by contracting blocks of muscles called myotomes that run along each side of the body. Because these muscle blocks are staggered, their contraction produces a side-to-side, undulatory movement that propels the animal through the water. They are able to swim both forward and backward.12

These animals are not buoyant and will sink quickly if they stop swimming. In addition to swimming, they are capable of moving rapidly through the substrate in which they live by burrowing into the sand. Their segmented muscle organization is thought to provide effective coordination for these locomotory movements.1234