It is the mouth of a hagfish, Class Myxini, the only craniates (animals with a skull) that do not also have a vertebrate column (a backbone). They lack jaws, and as you can see they have a pair of horizontally moving structures equipped with tooth-like projections made from keratin. When placed in a bucket of water and distressed, they can convert it into slime in seconds. I didn't quite believe it until I saw it for myself.