From Wikipedia:
"The wide mouth extends all around the anterior circumference of the head, and both jaws are armed with bands of long pointed teeth, which are inclined inwards, and can be depressed so as to offer no impediment to an object gliding towards the stomach, but to prevent its escape from the mouth. The anglerfish is able to distend both its jaw and its stomach (its bones are thin and flexible) to enormous size, allowing it to swallow prey up to twice as large as its entire body."
Improved Grab (Ex): To use this ability, an angler fish must hit a Huge or smaller opponent with its bite attack. It can then attempt to start a grapple as a free action without provoking an attack of opportunity. If it wins the grapple check, it establishes a hold and immediately traps the victim in its seizing maw.
Seizing Maw (Ex): Once the angler fish has successfully grappled an opponent, it automatically holds the victim in its mouth to slowly digest it. Each round that the victim spends in the angler fish's mouth, it takes automatic bite damage (and x points of acid damage?). To escape the hold, the victim must succeed on either a grapple check or an Escape Artist check opposed by the angler fish's grapple check. If the victim successfully escapes, it immediately takes maximum bite damage as it tears itself free of the angler fish's backward-curving teeth. An angler fish can hold a creature up to one size category lareger than itself within its mouth in this manner.