Over the last few years, the use of histological, histochemical and ultrastructural studies with scanning and transmission electron microscope become a good tools in Comparative Anatomy studies of some parts of the digestive system of different vertebrates.Three separate parts; buccal cavity, intestine and liver of fish and the tongue of birds were investigated. The investigation should help shed some light on these parts relating the structure to the function.These studies should be especially useful to comparative anatomists, histologists, pathologists, ornithologists and marine biologists.