Fishes have evolved some organs that mammals haven't, one of them being a combination of the liver and pancreas into a single organ, the hepatopancreas . This specimen in the image at left is from a fish, and the appearance is typical of the hepatopancreas. In this image the relationships of the pancreatic tissue to the liver and to the blood vessels are clearly seen. There is a duct system into which they empty, though you can't see it in this image. Not all fish have a hepatopancreas exactly like this, though most do.
Fishes are a very old group in evolutionary terms, by far the oldest group of vertebrates.
They have had so many millions of years in a fairly stable environment that adaptive radiation has produced some oddities in their anatomy and physiology that might well have been weeded out in a terrestrial setting. But the hepatopancreatic layout seems to have been the dominant theme.