Types of digestive systems
There re two types of digestive systems: incomplete and complete. An incomplete digestive system has only one opening to the digestive tract [link] a . Ingested food and excreted waste products pass through the same opening. Incomplete digestive systems are found in Cnidarians and Flatworms. The other type is, naturally, called a complete digestive system , and this type has two openings [link] b . Food is ingested through one opening, and waste products are excreted through a separate opening. This type of digestive system is found in all other phyla, including vertebrates.
The advantage of a complete digestive system is that it allows animals to feed continuously, without waiting for the residues of the previous meal to be released from the digestive tract. It also allows specialization of regions of the digestive tract, which means that different food types can be digested more efficiently. A particular region along the digestive tract, with different physical and chemical conditions, can be optimized for one type of nutrient and other regions can be optimized for efficient metabolism of other types of nutrients.
Invertebrate digestive systems
Invertebrates can have incomplete or complete digestive systems, as noted above. An incomplete digestive system has a gastrovascular cavity and only one opening for digestion. Platyhelminthes (flatworms), Ctenophora (comb jellies), and Cnidaria (coral, jelly fish, and sea anemones) have this type of digestive system. Gastrovascular cavities, as shown in [link] a , are typically a blind tube or cavity with only one opening, the “mouth”, which also serves as an “anus”. Ingested material enters the mouth and passes through a hollow, tubular cavity. Cells within the cavity secrete digestive enzymes that break down the food. The food particles are engulfed by the cells lining the gastrovascular cavity.
A complete digestive system, with an alimentary canal [link] b , is a more advanced system: it consists of one tube with a mouth at one end and an anus at the other. Earthworms are an example of an animal with a complete digestive system. Once the food is ingested through the mouth, it passes through the esophagus and is stored in an organ called the crop; then it passes into the gizzard where it is churned and digested. From the gizzard, the food passes through the intestine, the nutrients are absorbed, and the waste is eliminated as feces, called castings, through the anus.
Vertebrate digestive systems
Vertebrates have evolved more complex digestive systems to adapt to their dietary needs. Some animals have a single stomach, while others have multi-chambered stomachs. Birds have developed a digestive system adapted to eating unmasticated food.
Monogastric: single-chambered stomach
As the word monogastric suggests, this type of digestive system consists of one (“mono”) stomach chamber (“gastric”). Humans and many animals have a monogastric digestive system as illustrated in [link] ab . The process of digestion begins with the mouth and the intake of food. The teeth play an important role in masticating (chewing) or physically breaking down food into smaller particles. The enzymes present in saliva also begin to chemically break down food. The esophagus is a long tube that connects the mouth to the stomach. Using peristalsis, or wave-like smooth muscle contractions, the muscles of the esophagus push the food towards the stomach. In order to speed up the actions of enzymes in the stomach, the stomach is an extremely acidic environment, with a pH between 1.5 and 2.5. The gastric juices, which include enzymes in the stomach, act on the food particles and continue the process of digestion. Further breakdown of food takes place in the small intestine where enzymes produced by the liver, the small intestine, and the pancreas continue the process of digestion. The nutrients are absorbed into the blood stream across the epithelial cells lining the walls of the small intestines. The waste material travels on to the large intestine where water is absorbed and the drier waste material is compacted into feces; it is stored until it is excreted through the rectum.