Parts of a Sponge – Osculum, Pores and Collar Cells

All 11 parts on the diagram

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The sponge 4

Osculum The large opening where water leaves after passing through the central cavity. It is the way out, not a mouth.
Ostia (pores) The intake pores, where water enters the sponge. They lead through the wall to the cavity inside.
Spongocoel The central cavity of the sponge. In a simple sponge like this one it is lined with collar cells.
Spicules Stiff rods and spikes that make up the skeleton. Long straight ones stand round the osculum, and three-rayed ones lie in the wall.

The body wall 7

Pinacocytes Thin, flat cells packed tightly together in a single layer. They are the skin of the sponge.
Porocyte A cell shaped like a tube. Water passes through its middle, and the cell can tighten to slow the flow.
Mesohyl A jelly-like layer between the outer cells and the collar cells. Spicules and wandering cells lie in it.
Archaeocytes Cells that move about like an amoeba. They carry food to the other cells and can change into any other kind of sponge cell.
Choanocytes (collar cells) Collar cells whose beating flagella drive the water through the sponge. They catch the food it carries.
Collar A ring of tiny finger-like threads, the microvilli. It sits round the base of the flagellum. Food is caught here.
Flagellum A single long whip that rises from the middle of the collar. Its beating pulls water past the cell.

How this list was compiled: Every name checked on 2026-09-30 in two sources. Animal Diversity Web, University of Michigan, Porifera: pores (ostia), osculum, spongocoel, choanocytes (collar cells), mesohyl, spicules, and the simple asconoid sponge, https://animaldiversity.org/accounts/Porifera/. University of California Museum of Paleontology, Porifera, the cells: pinacocytes, choanocytes with their collar and flagellum, mesohyl, archaeocytes and porocytes, https://ucmp.berkeley.edu/porifera/pororg.html. Wikipedia, Sponge, Choanocyte, Pinacocyte, Mesohyl and Osculum, for a second reading of each name, https://en.wikipedia.org/wiki/Sponge. H. S. Pratt, A Course in Invertebrate Zoology (1902, public domain), for the needle-like spicules round the osculum and the three-rayed ones in the wall, https://archive.org/details/courseininverteb00prat. NOAA National Ocean Service, What is a sponge?, https://oceanservice.noaa.gov/facts/sponge.html. Licence: own work

The drawing: Drawn for this site on 2026-09-30 from plain geometry; nothing traced. Licence: own work

How it works

These are the parts of a sponge, the water animal and not the kitchen kind. First a simple vase-shaped sponge with half cut away, then a piece of its body wall enlarged to show the cells.

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The sponge

Water enters through the small pores, the ostia, all over the outside. It fills the central cavity, the spongocoel, and leaves by the one large opening at the top, the osculum. Needle-like spicules stiffen the wall and stand round the osculum. The arrows show the path of the water.

The body wall

Switch to Body wall. Flat pinacocytes cover the outside, collar cells line the cavity, and the jelly between them is the mesohyl, where archaeocytes wander. One collar cell is drawn large to show its collar and flagellum. Press Quiz me to name each part yourself.

The osculum is the way out, not a mouth. A sponge has no mouth at all.

Questions and answers

Is a sponge an animal or a plant?

An animal. Sponges are simple water animals, often mistaken for plants because the adults stay fixed in one place. They make up the phylum Porifera.

Do sponges have organs?

No. A sponge has no stomach, kidneys or other organs. Cells of different types do each job: collar cells move the water and catch food, and archaeocytes carry the food to the other cells.

How does a sponge eat?

The beating flagella of its collar cells pull water in through the pores. Food particles in the water are caught on the collars and taken into single cells, where they are digested.

What is the skeleton of a sponge made of?

Spicules and the protein collagen. The spicules are made of calcium carbonate or of silica, depending on the group of sponges.

What are the three body types of sponges?

Asconoid, syconoid and leuconoid. The drawing shows the simplest, an asconoid sponge: a plain tube with pores in its wall. In the other two the wall holds canals or small chambers lined with collar cells.