Glands located in the mouth that secrete fluids that moisten and lubricate the mouth and food and may initiate digestive activity; some perform other specialized functions. Fishes and aquatic amphibians have only solitary mucus (slime) secreting cells, distributed in the epithelium of the mouth cavity. Multicellular glands first appeared in land animals to keep the mouth moist and make food easier to swallow. These glands occur in definite regions and bear distinctive names. Some glands of terrestrial amphibians have a lubricative secretion; others serve to make the tongue sticky for use in catching insects. Some frogs secrete a watery serous fluid that contains ptyalin, a digestive enzyme. The oral glands of reptiles are much the same, but are more distinctly grouped. In poisonous snakes and the single poisonous lizard, the Gila monster, certain oral glands of the serous type are modified to produce venom. Also many of the lizards have glands that are mixed in character, containing both mucous and serous cells. Oral glands are poorly developed in crocodilians and sea turtles. Birds bolt their food, yet grain-eaters have numerous glands, some of which secrete ptyalin.