TentaclesA ring of soft arms round the mouth. The polyp uses them to catch small animals for food, to defend itself and to clear away debris.
MouthThe only opening of the body. Food goes in through it, and waste comes out the same way.
StomachA hollow that fills most of the body. It opens only at the mouth. Thin filaments inside it digest the food. It is also called the gastrovascular cavity.
Stinging cells (nematocysts)Tiny stinging cells in the outer tissue of the tentacles. Each holds a coiled thread that shoots out when prey touches it.
ZooxanthellaeTiny cells that live inside the tissues of most reef-building corals. They make food from sunlight, share it with the polyp and give the coral its color.
The cup and the skeleton 5
CoenosarcA thin band of living tissue that lies over the skeleton. It joins each polyp to its neighbors. It makes the coral one colony.
Calyx (cup)The cup of skeleton that a polyp builds and sits in. When it is disturbed, the polyp pulls itself down into the cup. This one is drawn empty.
Wall of the cupThe side of the cup. It stands round the lower part of the polyp like the wall of a well.
Basal plateThe floor of the cup. From time to time the polyp lifts itself and lays a new floor above the old one, and so the coral grows upward.
SkeletonThe hard part of a stony coral, made of calcium carbonate. The polyps lay it down under themselves, and over many years it builds a reef.
How this list was compiled: Every name checked on 2026-09-30 in two sources. NOAA National Ocean Service, Corals tutorial (What are corals?, Polyps up close, What is zooxanthellae?, How do stony corals grow?), https://oceanservice.noaa.gov/education/tutorial_corals/coral01_intro.html. Wikipedia, Coral, Corallite, Polyp, Zooxanthellae and Cnidocyte, https://en.wikipedia.org/wiki/Coral. Animal Diversity Web, University of Michigan Museum of Zoology, Anthozoa and Cnidaria, https://animaldiversity.org/accounts/Anthozoa/. Licence: own work
The drawing: Drawn for this site on 2026-09-30 from plain geometry; nothing traced. Licence: own work
How it works
What are the parts of a coral? A coral is built by small soft animals called polyps. Each polyp has a mouth, a ring of tentacles and a stomach, and sits in a cup of skeleton it makes itself.
Read the full guide
The polyp
Tentacles ring the mouth and carry stinging cells that catch food. The mouth is the only opening: food goes in and waste comes out through it. The stomach fills most of the body. The small rings in the body wall are zooxanthellae, which live inside the polyp.
The cup and the skeleton
A stony coral polyp builds a hard cup, the calyx, and sits in it. The floor of the cup is the basal plate. A thin band of living tissue, the coenosarc, joins each polyp to the next. Press Quiz me to name each part yourself.
The cup at the right is drawn empty so its shape shows. In a living coral a polyp fills every cup.
Questions and answers
Is a coral a plant or an animal?
An animal. A coral is made of polyps, soft animals related to jellyfish and sea anemones. Most corals are colonies of hundreds to hundreds of thousands of polyps.
What are zooxanthellae?
Tiny cells that live in the tissues of most reef-building corals. They make food from sunlight and share it with the polyp, and they give the coral its color.
What is a coral skeleton made of?
Calcium carbonate. Hard corals lay it down under themselves, and over many years the skeletons of many polyps build a reef. Soft corals make no rigid skeleton.
How does a coral polyp eat?
Mostly at night. Stinging cells on its tentacles catch tiny animals drifting past, and the tentacles move the food to the mouth.