Cephalochordates

Cephalochordates are a subphylum of chordates comprising small, soft-bodied marine animals commonly called lancelets or amphioxus. They inhabit sandy or muddy bottoms in shallow coastal waters where they remain mostly buried with just their anterior region exposed to feed. They are filter feeders that use mucus secreted by an endostyle to trap food particles drawn in by ciliary action.

Living cephalochordates belong to several genera, most notably Branchiostoma, Asymmetron, and Epigonichthys. The best-known species is Branchiostoma floridae, frequently used as a model in developmental and evolutionary studies. Cephalochordates are characterized by a slender, laterally compressed body, a notochord extending to the anterior end, a dorsal hollow nerve cord, segmented myomeres, pharyngeal slits, and a post-anal tail. They lack jaws and paired appendages, traits that distinguish them from vertebrates.

Ecologically, cephalochordates are benthic within marine environments, often remaining partially buried in sediment. They continuously filter water through the pharyngeal slits to capture microscopic food particles, aiding in nutrient cycling within shallow-sea ecosystems.

Reproduction is typically sexual, with external fertilization. The life cycle includes a free-swimming, tadpole-like larva that resembles a primitive chordate, followed by metamorphosis into the adult form.

Evolutionarily, cephalochordates are regarded as one of the most basal lineages within Chordata, retaining several primitive features that illuminate the early evolution of vertebrates and other chordates. The fossil record for cephalochordates is relatively sparse, but early Cambrian fossils show lancelet-like characteristics that inform our understanding of chordate origins.