Digestive System

The digestive system allows an organism to take in nutrients. It breaks the food down into particles that can be absorbed by the body. Aside from that, most organisms have different digestive systems that work differently depending on the organism. Each one has their own evolutionary advantages and disadvantages.

Earthworm

An earthworm's digestive system is very simple. It's digestive tract is divided into several different organs. The first is the pharynx, which sucks in the food. Next is the esophagus, which carries it to the crop. The crop stores the food and the gizzard grinds the food. Finally, the intestine digests and absorbs the nutrients and the waste it let out through the anus of the worm. This is the digestive system at it's simplest. However, it has it's disadvantages. The earthworm's intestine is very short, limiting the amount of food it can take in at once. The longer the intestine, the more food that organism can take in.

Crayfish

A crayfish's digestive system is slightly more advanced than the earthworm's. It's digestive tract consists of the foregut, midgut  and hindgut  The foregut conatins an enlarged stomach and the hindgut leads to the anus. With the digestive system, the crayfish is able to take in nutrients and let waste out.

Frog

The frog's digestive system begins with the mouth. First, they use their tongue to catch their prey. Then, their front teeth is used to grind their food before swallowing. The food moves through the esophagus and into the stomach, and shortly after, the small intestine. The small intestine digests the food and extracts the nutrients. When the food passes into the large intestine, the water and wastes are lead to the cloaca, where all wastes exit the frog. The frog has a slightly more advanced digestive system than the simpler organisms of earth, but it is still not as advanced as the rat's or human's digestive system.

Rat

The rat's digestive system is the most advanced out of all of the organisms. Like the frog, the rat begins digestion in it's teeth, where it crushes and tears food into smaller teeth. The food goes down the esophagus and into the stomach. The food then moves into the small intestine, where the intestine takes in nutrients from the ingested food. The villi and microvilli that covers the inside of the small intestine greatly increases the surface area of the small intestine and as a result, absorbs greater amounts of nutrients. At the end of the small intestine, the food travels to the large intestine, where the remaining water from waste material is removed by the large intestine. The waste leaves the rat from the anus and into the outside environment. The rat's digestive system is much like a human's digestive system. It can also absorb in a lot more nutrients at a time than rats can, giving it an evolutionary advantage.