In the elasmobranchs and agnathans the branchial
slits open individually to the outside,
although there are some exceptions. Outer folds
of skin create the vertical slit like opening to the
outside in the elasmobranchs and the branchiopores
of the agnathans (Fig. 1a,b,c). These openings
are generally lateral, although in the skates
and rays they are ventral.