This animal is called an obelia. You can see the digestive system that goes through the middle of its body.

Have you ever wondered how the food you put in your stomach can give energy to your fingers? Your stomach and your intestines break down the food you eat. Then, the little pieces of food travel through your blood to all the cells in your body. Each cell can absorb some of those pieces of food and use them to make energy!

This is a very complicated system, but it started out as something much simpler. Most animals have some kind of stomach - a pocket for their food. The stomach connects the outside where food is to the inside where the cells are. Things that go into the stomach are kept away from the cells until they are totally broken apart. This helps to make sure that only safe things get into the cells.

Over time more complex organisms, with more and more cells and a greater need for energy, appeared.

Extra Cellular Digestion

Some cells formed stomach-like pockets, which became special areas devoted to eating, like a kind of stomach.

In the obelia, to the right, we can see a special pocket or cavity. This pocket is lined with special cell including swallowing cells that release tiny chemicals to the pocket. These chemicals break down food as it comes in.

These different cells cooperate to break down and absorb the food, and then share the food energy with their neighboring cells. So, digestion started to occur more outside each cell, in the digestive pocket - or, extra cellular digestion.

Distribution System

As animals got larger, some cells became further and further away from the eating cells - and wouldn’t be able to get food in the simple way. So in larger animals, cells are found to have formed passageways, like human blood vessels, that spanned their whole bodies to distribute energy and oxygen to all their cells.

In the polychaete above, passageways like blood vessels transport food and oxygen.

The more an animal can eat and the faster and the more efficiently it can digest its food, the more energy an animal has to move around and seek food and a comfortable environment.

In order to eat faster and get even more energy, some animals’ stomach pockets elongated to become a passageway going through the center of their entire bodies—the gut or digestive tube!  This digestive tube allows the outside environment—the ocean and all its nutrients—to pass through the animal without being in contact with most of its cells

Other special pockets evolved around the digestive tube in humans, forming organs like the liver and the pancreas. The pancreas helps by making chemicals that help break down food in the intestine. Also, it makes insulin that helps cells absorb sugars really well.

The digestive tube, being very good at absorbing broken down food, is also a big point of entry for outsiders! So, lots of immune cells are found very close to digestive tubes. 

As organisms became more complex and specialized, with many different types of cells, the harder the immune system’s job became. Think of the human body, how many different cells and particles can you think of that belong to your body? Skin cells, hair cells, eye cells, calcium in your teeth, bone cells! So, the immune system needed to become more complex! New types of cells, like B cells and T cells, and tissues, like the lymphatic system, evolved to work together and be more efficient and specific. Once the immune system has discovered a particle or dead cell in your body, it calls a team of special swallowing cells that set to work breaking it down like food!