--- title: "Sponge Diagram: The Labeled Parts of a Sea Sponge" url: https://thesea.org/sponge-diagram/ date: 2026-11-13 modified: 2026-11-13 lang: en author: "Blane Perun" description: "A labeled sponge diagram explained: ostia, porocytes, spongocoel, osculum, choanocytes, pinacocytes, mesohyl, amoebocytes, spicules and spongin, and how water flows through a sea sponge." categories: - "Sponge Q&A" word_count: 1907 --- # Sponge Diagram: The Labeled Parts of a Sea Sponge [TheSea.Org](/)▸[Sponges](/sponges/)▸[Sea Sponge Facts](/sea-sponge-facts/)▸Sponge diagram # Sponge Diagram — The Labeled Parts of a Sea Sponge, Explained Sponges ▸ Q&A By [Blane Perun](/about-blane-perun/) November 13, 2026 10 min read What are the labeled parts of a sponge? A labeled sponge diagram shows: the **ostia** (small incurrent pores) formed by **porocytes**; the **choanocytes** or collar cells that pump water and trap food; the **spongocoel**, the central cavity; the **osculum**, the large opening where water leaves; the outer layer of flat **pinacocytes**; the jelly-like **mesohyl** in the middle, containing wandering **amoebocytes**; and the skeleton of **spicules** and **spongin** fibres. Water flows in through the ostia, past the choanocytes and out through the osculum. Quick facts Water entersOstia (incurrent pores) Water leavesOsculum (excurrent opening) Pump and filterChoanocytes (collar cells) Central cavitySpongocoel, or atrium SkeletonSpicules (silica or calcium carbonate) and spongin OrgansNone; sponges have no true tissues or organs 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](/sponges-in-reef-tank/) 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](/asconoid-syconoid-leuconoid-sponges/), but the same parts appear in all of them. Sponge diagram, labeled (asconoid sponge, cross-section) ↑ water out [ OSCULUM ] | | → o SPONGOCOEL o ← o = OSTIUM (pore in a POROCYTE) → o (central o ← inner lining = CHOANOCYTES → o cavity) o ← outer skin = PINACOCYTES |___________| wall middle = MESOHYL with holdfast AMOEBOCYTES and SPICULES 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](/what-does-sea-sponge-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](/barrel-sea-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](/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](/sea-sponge-structure/) covers the skeleton in detail. | Part | What it is | Job | | ---- | ---------- | --- | | Ostium | Small surface pore | Lets water in | | Porocyte | Tube-shaped cell forming a pore | Controls water entry in simple sponges | | Choanocyte | Collar cell with flagellum | Pumps water and traps food | | Spongocoel | Central cavity (atrium) | Collects filtered water | | Osculum | Large exit opening | Lets water out | | Pinacocyte | Flat outer cell | Covers and protects the sponge | | Mesohyl | Jelly-like middle layer | Houses mobile cells and skeleton | | Amoebocyte | Mobile, all-purpose cell | Transports food; becomes other cells | | Spicules | Mineral needles | Support and defence | | Spongin | Protein fibres | Flexible support | 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?](/sponge-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](/sponges-can-filter-an-amount-of-water-100000-times-their-size-each-day/). On a reef, that makes sponges one of the most important water-cleaning animals there is. Frequently asked questions What are the main parts of a sponge? Ostia, porocytes, choanocytes, spongocoel, osculum, pinacocytes, mesohyl, amoebocytes, spicules and spongin. What is the osculum of a sponge? The large opening, usually at the top, through which filtered water leaves the sponge. What do choanocytes do? They beat their flagella to pump water through the sponge and trap food particles on their collars. What is the spongocoel? The central cavity of a sponge where filtered water collects before leaving through the osculum. It does not digest food. Do sponges have organs? No. Sponges have specialised cells but no true tissues or organs. What are spicules made of? Either silica (glass) or calcium carbonate, depending on the type of sponge. How does water flow through a sponge? In through the ostia, past the choanocytes, into the spongocoel and out through the osculum. Sources & further reading - Brusca, R. C., Moore, W. & Shuster, S. M. (2016). *Invertebrates*, 3rd ed. Sinauer Associates. Chapter on Porifera. - Leys, S. P. & Hill, A. (2012). The physiology and molecular biology of sponge tissues. *Advances in Marine Biology* 62: 1–56. - World Porifera Database (WoRMS), classification of sponges. - University of California Museum of Paleontology, “Introduction to the Porifera”. Continue reading More from Sponges [Sponge Q&ASea Sponge AnatomyThe cells and canals that run a sponge. ](/sea-sponge-anatomy/) [Sponge Q&ASea Sponge StructureSpicules, spongin and skeletons. ](/sea-sponge-structure/) [Sponge Q&ASea Sponge FactsClassification, size, habitat, diet and lifespan. ](/sea-sponge-facts/) BP Last updated November 13, 2026 Written by Blane Perun Blane Perun has been exploring Earth’s oceans and marine life for over 25 years. Founder of TheSea.Org in 1999, underwater photographer, coral aquaculture pioneer, and explorer of 80+ countries. [About Blane Perun →](/about-blane-perun/) [More on Sponges →](/sponges/)