ABSTRACT
Recent molecular evidence that the hemichordates are extant sister group to the echinoderms is accepted and briefly discussed. Evidence is presented that the atria of tunicates are homologous with the otic vesicles of craniates. Primitive fossil chordates with calcitic skeletons (calcichordates) yield evidence of the origin and early history of the acustico-lateralis system. The first sign of the system to appear, in the stemgroup chordates known as cornutes, was homologous with the left ear but, not being invaginated, must have functioned as a lateral line. At the origin of the primitive crown-group chordates known as mitrates, the left ear was replicated on the right, as part of the phenomenon called mitrate organ pairing. At the same time, right and left ears were invaginated into atria, by a process which can be partly reconstructed, and became acoustic rather than lateralis in function. Then, in the stem group of the craniates, the right acoustic ganglion subdivided into a superficial ganglion, connected with a lateral line, and a deep ganglion in the right atrium, connected with the right ear. These two ganglia were initially in contact but later moved away from each other. The left ear, in the left atrium, did not subdivide and remained acoustic. This system, losing its asymmetries, evolved into the acusticolateralis system of extant craniates.
