Despite its many common characteristics, individual parts of the vertebrate digestive system can demonstrate a wide range of structural and functional variation, both within and among the classes of vertebrates. The headgut and hindgut show a wide range of adaptations to diet and habitat. The stomach is absent in some species of fish and serves as the principle site of microbial fermentation in some mammals. Some fish lack a distinct pancreas and the motor activity of the midgut differs between continuous and discontinuous feeders.  The hindgut is relatively short in fish and tends to be longest and most complex in terrestrial herbivores.  In addition to these structural differences, species can show wide variation in the degree of enzyme activity and some enzymes are absent from some species.

An understanding of these adaptations is essential for the proper care and maintenance of domesticated and captive animals, and the preservation of wildlife and endangered species. Comparative studies have also provided much of our present understanding of the basic physiological mechanisms and how they evolved.  This has led to a constant search for animals that can serve as good models for the study of human physiology and diseases.  The contribution of indigenous microorganisms to sodium, nitrogen, water, and energy conservation by the large intestine was first demonstrated in studies of the same phenomena in the ruminant forestomach.  Models for the study of disease are generally more obvious, due to similarities in signs, symptoms, or etiology.  However, some of the best animal models are the result of their apparent differences rather than similarities.  For example, the difference (simplicity) of non glandular frog skin from epithelium provided the basic model for determining the mechanisms of ion transport across the intestinal epithelium.