During
feeding, the velum is closed and the body juices
sucked from the host pass directly from the mouth
to the digestive tract without affecting respiration
and vice versa. This tidal mechanism also appears
to be the preferred route of water circulation in
free-swimming lampreys and differs from the
unidirectional flow through arrangement seen in
all other fishes (with the exception of the sturgeon;
Burggren and Randall, ’78).