The platypus does not have a stomach.

The platypus (Ornithorhynchus anatinus) is one of the few vertebrates that has completely lost its stomach through evolution. In most animals, the stomach uses hydrochloric acid and digestive enzymes like pepsin to break down proteins. However, genomic studies published in 'Genome Biology' in 2008 show that the platypus lacks the genes for these digestive functions. The esophagus of a platypus leads straight to the small intestine without any sac-like organ for storage or chemical processing. This anatomical quirk is also shared by its fellow monotreme, the echidna, and about 25% of teleost fish species. Scientists believe this loss occurred because the platypus's diet of soft-bodied invertebrates like worms and shrimp does not require heavy acidic digestion. Without a stomach to store food, a platypus must consume up to 20% of its body weight every single day. This requires the animal to spend roughly 12 hours a day foraging in the water. The transition from the esophagus to the intestine is marked by a simple valve, but the lack of an acidic 'holding tank' means digestion is a continuous and rapid process. This evolutionary regression is a rare example of a complex organ being lost because it was no longer energetically favorable to maintain. Researchers from the University of Adelaide have noted that the genes responsible for stomach acid (gastric proton pumps) are completely absent from the platypus genome. This confirms that the stomach didn't just shrink, it effectively disappeared from their biological blueprint millions of years ago.