Fish, amphibians, reptiles, birds and mammals all have a one-way digestive tract, as it is the most efficient and because they are more complex animals (when compared to invertebrates). The one-way digestive tract incorporates an entrance where the food is taken in and an exit like the anus where it leaves.
Ectotherms such as fish, amphibians, and reptiles all have a lower metabolism than endotherms such as birds, and mammals because they are not capable/dependent on internal generation of body heat.
Lampreys and Hagfishes are the most primitive type of fish and correspondingly do not possess a jaw, which restricts the type of food they can digest. Cartilaginous and bony fish, on the other hand, possess jaws that allow them to digest a variety of food since that allows them to have teeth. The jaws of fish evolved over time from the first pair of gill arches. Any type of muscles such as at the lips and cheeks stay absent. They have an esophagus that goes towards their stomach, and then an intestine that reaches the anus. In the class fish there are species that are carnivores, omnivores and herbivores.
Amphibians and most reptiles are carnivores and therefore have short and simple digestive tracts. Most Amphibians have very large and several liver lobes, an example being a frog. This is because it breathes primarily through its skin and therefore they will absorb all the toxins from the environment they live in (which in many cases is already an “unclean” area). They then get into their body and blood flow. Due to the fact that it is an overwhelming amount of toxins that enter the body the amphibian requires several lobes to produce enough bile for faster digestion of fat and filtering the blood from any pathogens or toxins coming from the food/environment. At the end of the digestive tract of an amphibian is the cloaca that is responsible for releasing excretory and genital products. The lining of the stomach of most amphibians like that of frogs has hard ridges and can be considered “fluffy” due to that. The hard ridges assist in mixing the food with the digestive fluids when the stomach muscles contract. The folds in the stomach lining also allows for the expansion while feeding and they increase the surface area, which allows for greater chemical digestion to occur. Many amphibians have developed a tongue that is not attached towards the back of the mouth, but rather near the chin. This allows them to flick their tongue out very quickly and catch their prey (mostly insects) very efficiently.
In reptiles the digestion is much slower than in mammals as they have a lower metabolism due to the fact that they are ectotherms. In reptiles, herbivores that lack teeth utilize stones to further break down and grind the food ingested just like birds. The digestive tract of herbivore reptiles is usually more complex because it has to digest fibrous parts of plants.
Birds lack teeth because they are too heavy and instead have a beak. To make up for the loss of teeth they ingest stones that act as teeth in the crop and assist in grinding and breaking down their food further. In their crop they have a strong acid just like all carnivores/predators to break down/digest meat.
Carnivores, herbivores, and omnivores are all found in the class Mammalia. Mammals have highly developed digestive system because of the amount they eat and the size they eat compared to other animals. Herbivores have a chambered stomach and long digestive tract with many microorganisms that allow them to break down the fibrous parts of plants. Even though certain large reptiles eat similar size animals as them they do not require such large amounts the whole time because they have a lower metabolism due to the fact that they are ectotherms. They have multiple structures, more than any other organisms and their mouth has very specialized teeth. These are adapted to their specific diet and shows of a complexity unlike other types of animals.