In the hagfish, water enters the individual
pouches from the pharynx medially via internal
branchial ducts and passes out laterally via an
external branchial duct (Fig. 1a,e). In the Bdellostoma
hagslime, the external branchial ducts lead
directly to the outside via individual branchiopores,
while in the Myxine hagfish the ducts
extend caudally to form a common external opening.
At the end of the series of gill pouches on the
left side only is the pharyngocutaneous duct that
connects the pharynx to the outside. This duct
serves as an entry point for respiratory water
when the hagfish has its head buried into its prey
(dead fish). Hagfish also have a nasal duct dorsal
to the mouth that opens into the pharynx and
serves as an important entry point for respiratory
water when the body of the hagfish is buried in the
mud.