Sea-urchins, Sand-dollars and Heart-urchins
Free-living Echinoderms with a globular, egg-shaped or flattened body. They have a compact, mostly hard test formed by regularly arranged plates carrying tubercles with movable spines.
The tube-feet are arranged in 10 meridian series, not in furrows. A double pore in the test corresponds to each tube-foot.
Mouth and alimentary canal
The intestine is long and coiled. The mouth is faced downwards towards the substratum.
The anal opening is always present.
- Order Clypeastroidea
- Order Diadematoidea
- Order Spatangoidea