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You've read throughout the species pages that penguins consume various marine animals, such as small fish and crustaceans, as food sources. This process of "foraging" is energetically expensive and can be dangerous. However, when they do feed, the food must be digested. Therefore, it travels through the penguin digestive system, which is a lot like the digestive system of all birds. It contains 6 primary components: esophagus, crop, stomach, gizzard, intestine, and cloaca. Digestive System
The oral cavity of the penguin contains a few structures that you are probably familiar with. It is covered by the bill or beak, which is a hard, keratinous structure. Penguin bills vary in shape and size, and this is dependent on the genetics of the penguin and how the penguin has evolved. Internal to the penguin bill is the oral cavity proper. Structures found within the oral cavity proper include the tongue with interesting keratinous protrusions emanating from it, and a cleft palate (roof of oral cavity). These keratinous spikes protruding into the back of the oral cavity help penguins hold on to fish or crustaceans that they capture when fishing for food. The cellular lining of these structures is stratified squamous epithelium, which are basically layers of flat cells that are designed to resist abrasion. The oral cavity leads into the esophagus, a structure which conducts food to the crop and eventually the stomach. The crop can store food for penguin chicks or for when the penguin wants a snack later on. Food partially digested in the stomach is delivered to the gizzard, a thick, muscular structure which grinds food into smaller bits. After this, the digested food is shuttled into the intestine to be absorbed into the body. The intestine of the penguin is similar to the intestine of the human being. The lining of the intestine is covered by numerous structures called villi. The picture below elucidates the basic villus. A villus is a projection of cells that increases the absorptive area of the intestine, and the cells covering the villus are renewed about once a week. Inside the villus are blood vessels and lymph vessels that shuttle digested nutrients into the body.
Enterocytes, or absorptive cells, take up sugars, proteins, minerals and liquids from the mix of digested fish or crustaceans penguins eat. After this occurs, sugars, proteins and minerals are conveyed into the capillaries and lacteals of the villus. Digested food particles enter the blood stream and are conveyed to the cells throughout the penguin. Here is a real picture of a penguin intestine (specifically ileum). You can see the villi on the right of the image and the muscular layers on the left of the image.
Food is digested and absorbed all along the intestine. The fate of undigested food or undigestible food is to end up excreted in the feces. Undigested food substance is deposited into the cloaca. A cloaca is a common dump found in penguins (and in embryological humans, even). Since this is a common dump, the excretions of the kidneys (nitrogenous waste in the form of uric acid) are deposited here. Additionally, when mating, sperm leaves the male's cloaca and enters the female cloaca. So, as you can imagine, the cloaca has a lot of functions. This concludes penguin histology |
All penguins form from a single cell, the zygote. Shortly into the development
of the penguin, the embryo folds over itself and forms a continuous canal with a single opening
and two close-off openings. The single opening leads directly to the yolk sac, and the other two
closed openings will form the oral cavity and the cloaca. This is the formation of the alimentary
canal, or digestive tract. Formation of this structure is better explained in the 
