The respiratory and feeding
mechanisms are separated internally by a modification
of the pharynx. The pharynx in the adult
lamprey is split into two ducts: one (dorsal)
leading to the esophagus and a ventral blindended
respiratory tube (branchial duct) that
adjoins the internal gill ducts. The flow of water
through the entrance of the respiratory tube is
controlled by a value, the velum (Fig. 1b).