Sponges are very simply structured ANIMALS. Sponges are the oldest multicellular organisms of all. They live glued to the seabed.
Sponges are like vases, with thousands of small holes. Water flows in through the small lateral openings, driven by many small cells with swirling cell tails. The water flows out again through the large opening at the top of the sponge. Sponges eagerly pump water through themselves and eat everything they can filter out of the water.

Sponges have a very simple structure. Sponges have no organs.

Sponges consist of only three different cell types, outer wall cells, inner cells and migrating midgut cells. The outer wall cells form the solid outer skin. Like human skin cells. The inner wall cells sit on the inner surface and use their flagella to pump the water towards the sponge and trap anything that floats past. These cells do what our intestinal cells do, except that the cells in the sponge’s inner skin do not receive the food pre-chewed, but have to catch it themselves first.

The third cell type is amoeba-like mobile. The cells scurry around in the body, bring food from the scavenger cells to the other cells and take care of the construction of the needle-like skeleton. Some of these cells act as sex cells. These mobile cells move in a jelly that is supported by the skeleton. The interior of the sponges is therefore a constant bustle of cells, all cells rush around like ants in an anthill.

Sponges do not yet have tissue in which one cell sticks to another. Sponges are somewhat looser multicellular organisms than we are. In us, almost all cells have their fixed place. Only a few human cells – such as blood and immune cells – drift around in our bodies.

What happens when a human being is put into a running blender? The person is broken down and dies. A sponge is different. If a sponge is stirred vigorously, all the cells separate from each other. Is the sponge then dead? No. The individual sponge cells float around, gather together and form a new sponge structure. Only the skeleton is gone and has to be rebuilt.

Sponges can reproduce sexually and asexually. Asexual reproduction takes place through the budding of new sponge structures. For sexual reproduction, the sperm are released into the sea. Some sperm end up in sponges of the same species, where they are not eaten but are transferred to the egg cells. The eggs fertilized in this way form buoyant larvae, which eventually set off in search of a good place to settle.

In the chapter „On genes and sex – The origin of the sexes“, the origin of the sexes is explained using the example of unisexual prehistoric sponges.