4.1 Digestion in the Mouth and Oesophagus
The hard surfaces of the mouths of most teleost; fishes would not lead one to expect any kind of secretion. However, many fish which chew with pharyngeal teeth or similar structures also produce mucus while chewing. Tests of this mucus in a few species for enzyme activity have so far yielded negative results. Likewise, oesophageal mucus cells, when examined histologically, showed no sign of containing any enzymatic granules, although there are reports of gastric-like secretory cells in the posterior oesophagus of a few fish.