The primary function of the digestive system is to deliver fuels and building blocks to sustain life and to eliminate wastes. A microcosm of bacteria, fungi, and archaea exists in the digestive tract of fishes. And while it is often considered to be its own system, its intimate ties to the digestive tract in terms of both location and function necessitate its inclusion as an associated organ. From a broad perspective, the digestive system is a simple tube connecting the organism to the environment, delivering supplies brought in via the buccal cavity and eliminating wastes via the anus, most simply illustrated by the gross anatomy of the hagfish digestive tract. Finally, the buccal cavity contains taste buds, acting as peripheral sense organs and transmitting information to the central nervous system, which show ultrastructural heterogeneity among taxa. The oesophagus is a short tube-like structure connecting the buccal cavity with the rest of the intestinal tract.