Fig. 1.

Vertebrate HBMs and the head–trunk interface. Human HBMs are composed of multiple voluntary muscles innervated by the hypoglossal nerve (XII) and cervical spinal nerves (C1–3) (a, top). The lamprey also possesses HBMs and the hypoglossal nerve (a, bottom). Schematic diagram of the head–trunk interface in gnathostomes (b, top) and the lamprey (b, bottom). Dotted lines indicate the interface. C1–3, cervical spinal nerves; g, gill opening; ot, otic vesicle; p, pharyngeal arches; so, somites