stomochord

The stomochord is a flexible, blind-ended tube-like structure found in hemichordates, most notably in acorn worms. It extends from the foregut into the proboscis region and has historically been interpreted as a possible homolog of the vertebrate notochord.

In embryology, the stomochord arises as an outgrowth of the foregut endoderm and projects into the proboscis coelom. It is lined by endodermal epithelium and differs from the vertebrate notochord in its origin and tissue composition, lacking a mesodermal rod. Its exact relationship to surrounding coelomic spaces varies among species.

Functionally, the stomochord’s role is not fully understood. It may contribute to the hydrostatic properties of the proboscis or play a passive structural role, but there is no consensus on a definitive function. It is not considered essential for locomotion in many species.

Evolution and classification: For much of the history of comparative anatomy, the stomochord was cited as evidence for chordate affinities, proposed as a notochord-like structure. Modern comparative anatomy and molecular data indicate that the stomochord is not homologous with the vertebrate notochord and likely represents a distinct endodermal outgrowth in hemichordates. This distinction informs discussions of chordate ancestry and the evolution of axial support structures.