The morphological and histological structure of the post-pharyngeal gut, liver and exocrine pancreas are described. In Poecilia the gut comprises only oesophagus and intestine and lies folded in the body cavity, while in Gasterosteus the straight gut also includes a true stomach.

In both species the oesophagus is lined by a stratified epithelium containing many saccular mucus cells. The oesophageal musculature consists of bundles of striated fibres.

The wall of the post-oesophageal gut comprises the following layers: simple epithelium, sub-epithelial connective tissue, circular and longitudinal smooth muscle, and serosa. A muscularis mucosae is not present in any part of the digestive tract in either species.

The stickleback stomach is lined by a simple columnar superficial epithelium. Gastric tubules, present only in the anterior half, are lined by pyramidal secretory cells of a single type, presumed to secrete both acid and pepsin.

In both species columnar absorbing cells possessing a striated border and mucussecreting goblet cells are the most numerous cell types in the intestinal epithelium.

The liver of each species is made up of tightly packed polyhedral hepatic cells with blood sinusoids running at random between them.

Both species have a diffuse pancreas. Acini sheath the mesenteric blood vessels. The choledochal and pancreatic ducts open together into the anterior intestine.