Stomach the first three chambers of which are all lined with oesophageal mucosa. In birds and crocodilians, the stomach is divided into two regions. Anteriorly is a narrow tubular region, the proventriculus, lined by fundic glands, and connecting the true stomach to the crop. Beyond lies the powerful muscular gizzard, lined by pyloric glands, and, in some species, containing stones that the animal swallows to help grind up food. In insects there is also a crop. The insect stomach is called the midgut. Information about the stomach in echinoderms or molluscs can be found under the respective articles. Stomach The stomach
Embryological origins of the mouth and anus that the structures are equivalent homologous, and that the older ideas about protostome mouth formation were correct. An alternative way to develop two openings from the blastopore during gastrulation, called amphistomy, appears to exist in some animals, such as nematodes. In many animals these early development stages later evolved in ways that no longer reflect these original patterns. For instance, humans have already formed a gut tube at the time of formation of the mouth and anus. Then the mouth is formed first, during the fourth week of development, and the anus is created four weeks later, temporarily forming a