Final answer:
The extensions of the calcareous plates that project through the skin of sea stars are called spines. These spines are connected to the ossicles and can be moved by small muscles for defense.
Step-by-step explanation:
The extensions of the calcareous plates that project through the skin of a sea star are called spines.
These spines are connected to the ossicles, which form the endoskeleton of the sea star. They can be moved by small muscles and locked into place for defense.
Some sea stars also have tiny stalked claws called pedicellaria, which are surrounding the spines and help keep the animal's surface clean.