Incomplete Digestive Systems
An incomplete digestive system has only one opening. The food goes in the same opening that the waste comes out. It would be as if your anus was the same opening as your mouth! Although this sounds gross, some animals make it work. Animals with this digestive system evolved early in time and are considered pretty primitive. They don't have the fancy digestive system or other organs that we have.
Let's look at some examples. Sea sponges may look like plants, but they are actually animals. The sea sponge filters water through its osculum, which serves as both its mouth and anus. Water goes in and special cells called choanocytes filter out food particles.
Other animals, such as jellyfish, have a more aggressive approach. You might be aware of the dangers of stinging jellyfish from your own ocean experiences. Tentacles with stingers stun or paralyze prey, allowing the jellyfish to consume eggs, small fish, and crustaceans. The food goes into the mouth, which is also where waste exits.
Complete Digestive Systems
Complete digestive systems have both a mouth and an anus, like humans. We call this type complete because it has a specific start and end point. Complete digestive systems were a huge advancement for animals. Food can be taken in continuously, and the animal does not have to wait for waste to exit before eating again. Food is also digested more thoroughly, using different techniques in different organs, such as acid in our stomach, and special proteins called enzymes in the stomach and intestines.
Since animals with complete digestive systems can get the most nutrients out of their food and keep eating, larger, more complex animals evolved. All mammals, like dogs, cats, and humans; reptiles, amphibians, fish, birds, and even insects have complete digestive systems.
Lesson Summary
In summary, the digestive system is a series or organs or cells that break down food into small, usable nutrients for animals. Incomplete digestive systems only have one opening for food intake and waste removal. These systems are more primitive and exist in lower animal species, like jellyfish and sea sponges.
Complete digestive systems are more complex and have a complete tube with separate openings for food and waste. Complete digestive systems allowed for more thorough food digestion and continuous food intake. Thus, more complex animals, like mammals, reptiles, fish, amphibians, birds and even insects have complete digestive systems.
