Here's why:
* Complete Digestive System: Animals with complete digestive systems have a digestive tract that is a continuous tube with two openings: a mouth and an anus. This allows for a one-way flow of food and waste.
* Lining: The inside of this tube, called the gut cavity, is lined with specialized tissues that help in digestion and absorption of nutrients. These tissues can include:
* Epithelium: A layer of cells that forms the lining of the gut.
* Muscles: Smooth muscle helps move food through the digestive tract.
* Glands: These secrete digestive enzymes and other substances.
Examples of animals with lined gut cavities (and complete digestive systems):
* Vertebrates: Including mammals, reptiles, birds, amphibians, and fish.
* Many invertebrates: Like annelids (earthworms), arthropods (insects), and mollusks.
Animals with incomplete digestive systems:
* Cnidarians: Like jellyfish and sea anemones, have a single opening that serves as both the mouth and anus.
* Platyhelminthes: Flatworms, also have a single opening.
Let me know if you have any other questions!