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In the higher vertebrates the regions of the tract comprise the oesophagus, stomach, small intestine, large intestine and rectum. In ruminant herbivores (e.g. the ox) the stomach is very large and divided into four distinct regions, each with its own structure and function (see Sections 4.1 and 4.2.). The small intestine comprises the duodenum, jejunum and ileum (although these two divisions are ill‐defined and are termed the jejunoileum). The large intestine in most species comprises a fairly long portion known as the colon, with a relatively short blind‐ending diverticulum known as the caecum. However, there are wide species variations, and in some herbivores (e.g. the horse), the caecum is very large (see Section 6.2.1).

In lower vertebrates marked variations from the above pattern occur. For example, in several varieties of fish no stomach is present, there being only a single tube between the pharynx and the intestine, which may be regarded as equivalent to the oesophagus. In fact the oesophagus only becomes well defined structurally in land vertebrates with the development away from gill breathing, i.e. an adaptation for the extraction of oxygen from water.