The way of eating and the type of food consumed by animals largely conditioned the development of the digestive system and digestive structures. Invertebrates have simpler digestive structures, most often made up of the mouth and gastrovascular cavity or primitive forms of the intestines and stomach. Higher vertebrates are especially at the center of the observation of digestive adjustments that have arisen as a result of different types of diet. According to the type of diet, the division into herbivores, carnivores and omnivores is common. Herbivores have the most complex digestive structures as plant foods are very difficult to digest. Carnivores, on the other hand, have simpler digestive system. Digestive structures that are mostly modified due to diet are stomach, intestines and teeth. It is believed that the beak shape in birds is the result of diet, but also the action of some other environmental factors. In large number of species, structure named crop helps them to mechanically process whole bites of food. It has been observed that some carnivorous species can eat and effectively digest plant foods without special structures – with help of fermenting bacteria. Herbivorous species cannot break down meat because they do not have the enzymes that would allow them to do so. In addition to anatomical, animals also adapt to various food on histological level. Tissues contain numerous cells that secrete digestive enzymes and other excipients such as mucus, which are essential for the normal digestion. There are also salivary glands in the mouth from which digestion actually begins, and goblet cells that help form and facilitate the formation of feces that the body eliminates from the body.