Gill Pouches

or gill pockets, paired protrusions of the pharyngeal wall, present in all vertebrate animals in the early stages of their development. In cartilaginous fish, the spaces between the gill plates are sometimes called gill pouches. In cyclostomes the gill pouches are part of the gill apparatus that fulfills the function of respiration; there are from six to 14 pairs of them. Each gill pouch is spherical; its mucosa forms folds—the gill lobes. The gill pouches unite with the pharynx (in hagfish) or with a special respiratory tube (in lampreys) by means of an internal canal. The external canals of the gill pouches open to the body surface separately or after prelimi-nary merging (in hagfish).