The influences which bring about such derivations and modifications may be enumerated as follows: 1. The habitual amount of food required by the animal. 2. The volume and digestible character of the food. 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: =1. Stomach Forms Depending on the Influence exerted by the Habitual Amount of Food required by the Animal.=--The greater the activity of tissue changes is, the greater will be the amount of food required and the more pronounced will be the gastric dilatation of the alimentary canal. Hence in the higher vertebrates generally the stomach appears as a large and more sac-like dilatation than in lower forms, such as fishes and amphibia and some reptilia, in which the stomach is usually smaller and foetal in shape, forming a slight longitudinal dilatation situated in the long axis of the body. An example is seen in the stomach of _Coluber natrix_ (Fig. 52). Frequently this slight dilatation is scarcely differentiated from the oesophagus at the cephalic and from the small intestine at the caudal end. Many batrachians and perennibranchiates possess this form among the amphibia. It is also encountered in the pickerels, the _Cyprini_, and in _Labrus_ among fishes, and in some saurians and ophidia among reptiles. It constitutes a slight advance in development over the earliest stage represented, as we have seen, by the nearly uniform and undifferentiated alimentary tube of amphioxus and the cyclostomata. [Illustration: FIG. 52.--Alimentary canal of _Coluber natrix_. (Nuhn.)] This transition of the foetal form to the more advanced secondary types of the stomach is marked by the development of two important structural features: (_a_) The separation in the interior of the canal of the stomach from the intestine by the appearance of a ring-shaped valve, the _pyloric valve_. This is produced by an aggregation of the circular muscular fibers of the intestine at this point, and causes a projection of the mucous membrane into the lumen of the canal. It begins to appear in the fishes (pickerel, sturgeon, etc.), is found in most amphibia and is regularly present in the stomach of the higher vertebrates. (Figs. 54 and 55.) A good example of the ring-shaped plate of the pylorus with central circular opening produced by the aggregation of the circular muscular fibers is afforded by the view of the interior of the cormorant's stomach given in Fig. 69. The opposite or oesophageal extremity of the stomach is less well differentiated from the afferent tube of the oesophagus.