Animals ingest their food through their mouths. Now, hopefully, it will come as no surprise that something as essential as the ingestion of food will be affected in a very significant way by natural selection. So, that is why there is a wide variety of forms and functions in terms of mouth parts. You can see here, this is the skull of a deer and they have these nice grinding teeth, which are useful when you're an herbivore and you need to grind up plant matter. Whereas this saber-toothed cat here, not going to be doing a lot of eating of plants, and hopefully, you know, you can tell what these bad boys are for. And of course, Darwin's moth, which has a very specialized feeding tube for a very special flower that has a very deep nectary that only it can reach. Now because natural selection has such a powerful effect on mouth parts, you'll often see some interesting examples of adaptive radiation with mouth parts. Adaptive radiation, remember, is when an organism diversifies and gives rise to a bunch of different variations, due to trying to fill different niches. A great example of this is the finches that Darwin saw on the Galapagos, which all had these different types of beaks depending on their food source. They all came from this one common finch ancestor, but on the islands due to all the niches to fill their mouth parts evolved to have the form fit the function.
Now, nutrients are going to be absorbed in a 4-step process. The first step is ingestion, bringing food into the body, specifically into the digestive tract. Then you have digestion, which is the breakdown of the food through both chemical and mechanical, or like physical means, like chewing. Right? Grinding your Mechanically digesting the material. And it should be noted that humans perform what's called extracellular digestion, as do lots of other animals. However, some organisms will perform intracellular digestion, where they actually bring the material into their cells to be digested. Whereas we do it in the lumen or the hollow space inside of our digestive organs, which of course is outside of the cells. Now, you also need to do something with the nutrients you extract from digestion, and that is absorption, where you absorb them into the body to make use of them. Lastly, you're not going to use everything you take in, there's going to be some waste. So you have to eliminate it, eliminate that undigested material in some manner that we need not discuss here.
Now, there are going to basically be digestive tracts that fall into 2 categories. Incomplete digestive tracts, where there's a single opening that food enters, and waste exits. A nice example of this is the gastrovascular cavity of Cnidaria and Platymenesthes, which, you can see 2 cnidaria here. We have a hydra and some type of medusa, you know, jellyfish-like creature, and they just have this, one, I'm just going to write, you know, GVC for gastrovascular cavity. So, basically food is going to go in there, it's going to get digested, that's what's happening, this food's getting digested. And then it's going to exit through the same opening. We, on the other hand, have what we commonly call an alimentary canal, but is technically referred to as a complete digestive tract, and that is to say that it has two openings. Our food enters through the mouth, works its way through this digestive system, which we're going to go through momentarily. So, it's going to make its way through the esophagus here, into the stomach right here. From the stomach, it enters the small intestine, wiggles all its way through the long small intestine until it hits this area, the large intestine, where it finally is formed into feces and will exit through the anus. So, that is the whole of the process of digestion through a complete digestive tract. Let's actually flip the page and take a closer look at this whole organ system.