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Complete vs. Incomplete Digestive Systems

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Instructor: Amanda Robb

Amanda has taught high school science for over 10 years. She has a Master's Degree in Cellular and Molecular Physiology from Tufts Medical School and a Master's of Teaching from Simmons College. She is also certified in secondary special education, biology, and physics in Massachusetts.

Explore and compare complete and incomplete digestive systems, including what these terms mean and what organisms have each type. Gain a greater understanding of how digestive systems work and the way complete and incomplete digestive systems form. Updated: 11/29/2021

What Is the Digestive System?

As you're watching this video, you might be having a snack for some brain food. What actually gets this snack to your brain though? Before we transport the delicious nutrients to our brain cells, we have to first break them down into usable parts. This is where the digestive system comes in.

The digestive system is a collection of organs or cells in an organism's body that breaks down food into smaller nutrients that the body can use. Our digestive system has an opening and an exit, meaning it is a complete digestive system. However, there's another type you might be less familiar with, which is the incomplete digestive system. Let's take a look at each of these systems in more detail!

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  • 0:00 What is the Digestive System?
  • 0:41 Incomplete Digestive System
  • 1:36 Complete Digestive Systems
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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.

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