Substances in solution can be tested by taste buds, there are four fundamental sensations of taste, they are sweet, salty, bitter, and sour.
Taste buds are widely distributed in fishes, being found in the mouth, pharynx, branchial cavities, and outer surface of the head, in some fishes they occur on the entire body surface and the sensation of taste is used to detect substances in the environment, the taste buds are innervates by the V, VII, IX and X cranial nerves.
In other vertebrates the taste buds are confined to the tongue, oral cavity, and pharynx being innervated by the VII and IX cranial nerves, the X cranial nerve may have a few connections with taste buds.
In frogs the tongue has filiform (conical) and fungi form (knob-like) papillae, but taste buds are found only on fungiform papillae.
In birds the horny tongue has no taste buds, they are found in the lining of the mouth and pharynx.
In mammals there are various kinds of papillae on the tongue, they are filiform papillae which are small conical elevations, often cornified but bearing no taste buds; foliate papillae are leaf-like structure running in parallel rows and bearing taste buds along their sides; fungiform papillae are round knob-like elevations with a few taste buds on their surface; and circumvallate papillae are few but large rounded structures at the base of the tongue having taste buds along their side walls.
Some taste buds of vertebrates are not for tasting but for testing substances in the pharynx to cause reflexes which prevent solid particles from entering air passages.