The first part of the small intestine. At the junction of the stomach and the duodenum the alimentary canal is inflected. The duodenum first goes anteriorly for a short distance, turns dorsally, and eventually caudally, thus it is a U-shaped structure with two horizontal sections (a ventral and a dorsal one).
Comment:
[taxon-note] "In fish, the divisions of the small intestine are not as clear, and the terms anterior intestine or proximal intestine may be used instead of duodenum." xsd:string {scope="NCBITaxon:32443", source="Wikipedia:Duodenum"}; [taxon-note] "In humans, the duodenum is a hollow jointed tube about 10-15 inches (25-38 centimetres) long connecting the stomach to the jejunum. It begins with the duodenal bulb and ends at the ligament of Treitz." xsd:string {scope="NCBITaxon:9606", source="Wikipedia:Duodenum"}