Sponges are the simplest animals most people will ever see, but their bodies are cleverly built. A sponge is essentially a living water pump and filter. Every part of a sponge diagram is there to move water through the body, catch food from it and keep the whole structure standing. Once you understand the flow of water, the labels make sense. This page walks through a standard sponge diagram part by part, in the order water travels through the animal, then covers the cells and the skeleton.

If you keep a reef aquarium with live rock, you will probably meet sponges without ever buying one. Small encrusting and tube sponges hitchhike in on rock and grow in shaded corners and under ledges, and watching one is a good way to see these parts in real life. I have used live rock in every reef system I’ve built since 1996, and the life that comes in on it is one of the joys of the hobby; our page on sponges in reef tanks covers what to expect.

The basic sponge diagram

Most textbook diagrams show the simplest body plan, called asconoid: a hollow vase or tube with a thin wall.

Picture a small vase standing on a rock. Its wall is peppered with tiny holes. Water seeps in through those holes into the hollow centre and flows out through a single larger opening at the top. The inside of the wall is lined with collar cells that drive the current and catch food. That is the whole design. Larger sponges fold and thicken the wall into networks of canals and chambers, giving the syconoid and leuconoid body plans covered in our guide to sponge body types, but the same parts appear in all of them.

Ostia and porocytes: where water gets in

The ostia (singular ostium) are the many small pores on the surface of the sponge through which water enters.

The name of the whole phylum, Porifera, means “pore bearers”, after these openings. In the simplest sponges, each ostium is a channel through a single tube-shaped cell called a porocyte, which can contract to narrow or close the pore. In more complex sponges, the incurrent pores lead into a system of canals rather than straight into the central cavity. Ostia are usually too small to see clearly with the naked eye, though on a large sponge you can often see a fine, velvety texture created by thousands of them.

Choanocytes: the pump and the filter

The choanocytes, or collar cells, are the most important cells in a sponge. They line the inner cavity or the internal chambers and do two jobs at once.

Each choanocyte has a single whip-like flagellum surrounded by a ring, or collar, of tiny finger-like projections called microvilli. The beating flagella of thousands of choanocytes create the current that pulls water in through the ostia and pushes it out through the osculum. As water passes through the collars, bacteria and tiny particles of organic matter are caught on the microvilli and taken into the cell to be digested or passed on. Choanocytes look remarkably like single-celled organisms called choanoflagellates, which are among the closest living relatives of animals, and that similarity is one of the clues to how animals first evolved. Our page on what sea sponges eat explains the feeding in more detail.

Spongocoel and osculum: where water goes out

After passing the choanocytes, water collects in the spongocoel, also called the atrium, and leaves through the osculum.

The spongocoel is the central cavity of a tube or vase sponge. It is not a stomach, because no digestion happens there; it is simply a collecting space for filtered water. The osculum (plural oscula) is the large opening, usually at the top, through which the water exits. A big sponge may have many oscula. The outflow is often surprisingly strong, which helps carry used water and waste well away from the sponge so it does not re-filter it. Some sponges can narrow the osculum using contractile cells around its rim, slowing or stopping the flow. If you have seen a giant barrel sponge on a reef, the huge opening at the top is its osculum and the barrel’s interior is the spongocoel.

Pinacocytes and the mesohyl: the body wall

The sponge’s wall has an outer skin, an inner lining and a jelly-like layer between them.

The outer surface is covered by flat, tightly packed cells called pinacocytes, which together form the pinacoderm. They protect the sponge and can contract slightly to change its shape. Between the outer and inner layers lies the mesohyl, a gelatinous matrix of collagen in which several kinds of mobile cells live. The most important are amoebocytes, also called archaeocytes, which crawl through the mesohyl distributing food, carrying away waste and, crucially, turning into any other type of sponge cell when needed. Other mesohyl cells, sclerocytes and spongocytes, build the skeleton. The mesohyl is where most of the sponge’s “living” happens, even though it looks like nothing more than jelly. For a deeper dive into the cell types, see sea sponge anatomy.

Spicules and spongin: the skeleton

A sponge’s skeleton is made of tiny mineral needles called spicules, flexible protein fibres called spongin, or both.

Spicules come in many shapes, from simple rods to stars, anchors and three- or four-rayed forms, and they are made either of silica (glass) or calcium carbonate, depending on the sponge’s class. Calcareous sponges have calcium carbonate spicules; glass sponges have six-rayed silica spicules that can fuse into stunning lattices; demosponges, the largest group, usually have silica spicules, spongin or both. Spongin is a tough, collagen-like protein; the soft, springy skeleton of a natural bath sponge is pure spongin with no spicules. Spicule shapes are so varied and characteristic that taxonomists often use them to identify species under the microscope. Our page on sea sponge structure covers the skeleton in detail.

How to draw and label a sponge diagram

If you need to draw a sponge diagram for a biology class, a few simple rules will get every label in the right place.

Start with a vase or tube shape attached to a base, and draw it in cross-section so the hollow middle shows. Mark the opening at the top as the osculum and the hollow middle as the spongocoel. Draw the wall as two thin layers with a thicker band between them. Label the outer layer pinacocytes, the inner lining choanocytes (draw a few as small cells with a collar and a single flagellum pointing into the cavity), and the middle band mesohyl. Put a few irregular blob-shaped cells in the mesohyl and label them amoebocytes, and add some needle shapes for spicules. Finally, draw small channels through the wall and label them ostia, with a tube-shaped porocyte around one of them. Use arrows to show water entering through the ostia and leaving through the osculum, because examiners often want to see the direction of flow.

Two common mistakes are worth avoiding. The first is labelling the spongocoel as a stomach or gut; it is only a passage for water. The second is drawing choanocytes on the outside of the sponge; they always face the water channels inside. If you are asked to compare body types, show the asconoid sponge first and then explain that syconoid and leuconoid sponges fold the choanocyte layer into canals and chambers to increase the pumping surface.

Tracing the flow of water

The simplest way to remember a sponge diagram is to follow a drop of water through it.

Water enters through the ostia on the surface, passes through porocytes or incurrent canals, and reaches the choanocytes, whose beating flagella drive it on and whose collars strip out food. It then collects in the spongocoel or in excurrent canals and leaves through the osculum. Oxygen is taken up and carbon dioxide and waste are released along the way, since a sponge has no lungs, gills or kidneys; see do sponges have a circulatory system? The volume is remarkable. Some sponges can pump many times their own body volume of water every hour; see how much water sponges filter. On a reef, that makes sponges one of the most important water-cleaning animals there is.