The oesophagus of eels (Anguilla) is an exception to this general pattern. It is relatively long, narrow, and serves during seawater residence to dilute ingested seawater before it reaches the stomach. A possible conflict between the osmoregulatory and digestive roles of the gut in marine fish in general will be discussed later (Section 3.5).
The transport of food from the stomach into the midgut is controlled by a muscular sphincter, the pylorus. The control of the pylorus has not bean demonstrated in fish, but the best guess at this time is that it resembles that in higher vertebrates. The pylorus is developed to various degrees in different species for unknown reasons, in some species even being absent. In the latter case, the nearby muscles of the stomach wall take over this function, which may also include a grinding function by the roughened internal lining. In fish which lack a stomach, the pylorus is absent and the oesophageal sphincter serves to prevent regress of food from the intestine, i.e., in fish lacking a stomach and pylorus, the midgut attaches directly to the oesophagus.