Tongue:
A tongue develops in the floor of oral cavity in many vertebrates e.g. lamprey has a prototractible tongue with horny teeth that rasp its prey flesh. Fishes may have a non-muscular tongue that bears teeth that help hold prey. Tetrapods have evolved mobile tongues for gathering food. Frogs and salamanders and some lizards can rapidly project part of their tongue from the mouth to capture an insect. A wood pecker has a long spiny tongue for gathering insects and grubs. Ant and termite eating mammals also gather food with long, sticky tongues, spiny papillae on the tongues of cats and other carnivores help these animals rasp flesh from a bone.
Salivary glands:
Most fishes lack salivary glands in the head region. Lampreys are an exception because they have a pair of glands that secrete an anticoagulant needed to keep their preys blood flowing as they feed. Modified salivary glands of some snakes produce venom that is injected through fangs to immobilize prey.
In amphibians or reptiles and must birds lack salivary glands while all mammals have them.
Esophagus:
Esophagus is short in fishes and amphibians but much longer in amniotes due to their longer necks. Grain and seed eating birds have a crop that develops from the caudal portion of the esophagus. Storing food in the crop ensures continuous supply of food to the stomach and intestine for digestion. This structure allows these birds to reduce the frequency of feeding and still maintain high metabolic rate.
Stomach:
Stomach is an ancestral vertebrate structure that evolved as vertebrates began to feed on longer organisms that were caught at less frequent intervals and required storage. The gastric glands and their production of Hydrochloric acid (HCl) evolved to kill bacteria and helping preserve food. The enzyme pepsinogen has evolved later because the stomach is not essential for digestion.
Gizzards:
Some fishes, crocodilians and all birds have a gizzard for grinding up food. The bird’s gizzard develops from the posterior part of stomach called the ventriculus. Pebbles that have eating birds and facilitate the grinding process.
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