This lesson is on the digestive and excretory systems of porifera. In this lesson, we’ll go over what a poriferan is and the basic cells needed to digest food and excrete waste.
What Are Porifera?
A glance under the sea while snorkeling, or even in the pages of National Geographic, might reveal a colorful forest of plants and animals. However, the organisms you might quickly identify as plants are surprisingly animals.
Large, colorful barrels and branching tree-like structures attached to rocks aren’t plants after all. These sessile, non-moving, organisms are called porifera.Porifera are a group of the most basic animals.
They are made of four types of cells bound together with a gel-like substance called mesenchyme. Other, more evolved animals have collections of cells that all work together in tissues, which make organs and organ systems. These complex structures are absent in porifera.Think of animal species like businesses. Businesses usually start pretty simple. Even giants like Apple were once just a few employees in one building. Some small, simple companies, like maybe a local sandwich shop, stay the same.
Each person does a few different jobs with maybe one main focus. They make a profit and do just fine. Likewise, the porifera sea sponge are simple creatures with only a few cell types, and have survived for hundreds of millions of years.Some businesses get more and more complex. Companies like Apple or Microsoft now have dozens of departments, each with department heads and different people working under them, all over the world. Likewise, humans have evolved considerably since their simple start, and are now complex creatures with over 200 different cell types.But, both companies still make a profit and stay out there in the business world.
The porifera certainly have been around for billions of years on Earth, so they must have processes that work well. Let’s take a look at two of them, digestion and excretion.
Digestion is the process of taking in food and breaking it down to get energy. Porifera are filter feeders, meaning they take in water and extract their prey from it, instead of actively going after prey like a fish or shark. Water flows in through incurrent canals, which are all holes over the outside of the poriferan, like the holes you see in your kitchen sponge.
Then, the water flows out through a central opening at the top, called the osculum. Let’s look closer at what cells are involved in this process next.The incurrent canals are lined with specialized cells called choanocytes, which help with water flow and to bring in prey. They have long protrusions called flagella that beat like a whip back and forth, creating a current to bring in water. Choanocytes have a collar on their outer surface that acts like a net to capture their prey, plankton.
Once the prey is in the net, the choanocyte engulfs it and distributes it to another type of cell called an archaeocyte, also called an amoebocyte. Since all cells in the poriferan need food, the job of the archaeocytes is to take the plankton from the choanocytes, break them down, and then give the food needed to other cells in the body.Think of it like a food processing company for fish. First, the fish are caught and brought to the factory, like how the choanocyte catches the plankton. Then, the fish need to be distributed through trucks all around the city to the individual stores where the fish is needed, like the archaeocytes distributing to other cells.
Excretion happens after the poriferan has used all the food and needs to get rid of the waste.
When cells use food to make energy, they make waste products, like carbon dioxide and urea from protein metabolism. These metabolic waste products flow out of the cell into the surrounding water through diffusion, where things flow from a high concentration to a low concentration.Once in the water, the waste products are moved by the current through that outer opening, the osculum. The most simple sponges only have one osculum, but more complex poriferans can have many, leading to larger porifera.
In summary, porifera are sessile, aquatic animals. Due to their early evolution with little change since, they have a very basic body structure with only four types of cells sandwiched together. Their digestive system consists of a number of incurrent canals that allow water to flow into the central cavity of the poriferan. As water flows in, choanocytes capture prey like plankton in their collars. Archaeocytes engulf the prey from the choanocytes, digest it, and distribute nutrients to other cells in the poriferan. Any metabolic waste products leave through a central opening at the top of the poriferan called the osculum.