The Lancelet and Ammocoete Mouths

The evolutionary history of the vertebrate mouth has long been an intriguing issue in comparative zoology. When the prevertebrate state was considered, the oral structure in adult lancelets (amphioxus) was traditionally referred to because of its general similarity to that of the ammocoete larva of lampreys. The larval mouth in lancelets, however, shows a peculiar developmental mode. Reflecting this, the affinity of the lancelet mouth has long been argued, but is still far from a consensus. The increase in available data from molecular biology, comparative developmental biology, paleontology, and other related fields makes it prudent to discuss morphological homology and homoplasy. Here, we review how the lancelet mouth has been interpreted in the study of evolution of the vertebrate mouth, as well as recent advances in chordate studies. With this background of increased knowledge, our innervation analysis supports the interpretation that the morphological similarity in the oral apparatus between ammocoetes and lancelets is a homoplasy caused by their similar food habits.