Sea urchins
Sea urching belong to the phylum of Echinodermata. Just like other echinoderms they have a radial symmetry of five. The shell consists of grown together chalk elements. The spines could move, and were attaches to the chalk elements. Sea urchins live on the seafloor.
The shell of sea urchins often in well preserved. The shell is made of calcite. The spines usually fall of and are found separately. Sea urchins first occurred in the Ordovician  period.
Example of a Sea urchin.
There are two types of sea urchins. The regular sea urchins are totally five radial symmetrical. Their anus is on top, and their mouth is on the bottom. Irregular sea urchins have their mouth and anus both on the bottom. There sea urchins are less symmetrical.
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