Final answer:
Spicules are needle-like structures found in the skeleton of sponges that provide support and protection. They can be made of calcite or silica and help differentiate between different classes of sponges.
Step-by-step explanation:
Spicules are small needle-like structures found in the skeleton of sponges that provide support and protection. They are made of calcite or silica and are part of the sponge's endoskeleton. Sponges can have different types of spicules, such as siliceous, calcium carbonate, or spongin, depending on their class. The presence and composition of spicules are used to differentiate between different classes of sponges.