All species have large,
telescoping eyes which dominate and protrude from the
skull, but are enclosed within a large transparent dome of soft tissue.
[4] These eyes generally gaze upwards, but can also be directed forwards.
[5] The opisthoproctid eye has a large
lens and a
retina with an exceptionally high complement of
rod cells and a high density of
rhodopsin (the "visual purple" pigment); there are no
cone cells. To better serve their vision, barreleyes have large, dome-shaped transparent heads; this presumably allows the eyes to collect even more incident light and likely protects the sensitive eyes from the
nematocyst (stinging cells) of the
siphonophores from which it is believed the Barreleye steals food. It may also serve as an accessory lens (modulated by intrinsic or peripheral muscles), or refracts light with an index very close to seawater. A recent study disclosed that
Dolichopteryx longipes is the only vertebrate known to use a mirror (as well as a lens) in its eyes for focusing images.
[6]
The toothless mouth is small and terminal, ending in a pointed snout. As in related families (e.g.
Argentinidae), there is an epibranchial or
crumenal organ present behind the fourth
gill arch. This organ—analogous to the
gizzard—consists of a small
diverticulum (pouch) wherein the
gill rakers insert and interdigitate for the purpose of grinding up ingested material. The living body of most species is a dark brown covered in large, silvery imbricate
scales; but these are absent in
Dolichopteryx, leaving the body itself a transparent white. In all species a variable number of dark
melanophores colour the muzzle, ventral surface, and midline.
