1. The habitual amount of food required by the animal.
2. The volume and digestible character of the food.
Fig. 52.—Alimentary canal of Coluber natrix. (Nuhn.)
3. The size and shape of the abdominal cavity in which the stomach is contained.
4. Structural modifications designed to increase the action of the gastric juice on the food contained in the stomach.
5. The assumption, on part of the stomach, of functions which are usually relegated to other organs.
Most of the individual stomach forms encountered among vertebrates owe their production to several of these influences acting in conjunction.
We may group the main types as follows: