Ernst Haeckel described a taxon comprising tunicates, cephalochordates, and vertebrates in 1866. This evidence suggests that IRGs may function in an immune-related capacity in cephalochordates. The segments resemble the v-shaped muscle blocks found in similar cephalochordates such as "Amphioxus". New phylogenomic analysis shows urochordates, rather than cephalochordates, as the sister group to vertebrates. Cephalochordates are represented in modern oceans by the Amphioxiformes (lancelets, also known as amphioxus).