Cidaroids are unique among echinoids in having thick spines that are not covered in soft tissue. The 'dead' surfaces of their spines offer an ideal substrate for encrusting algae and small organisms to grow on, providing a natural camouflage. In the picture above the regular echinoid Eucidaris (arrowed) is almost perfectly hidden among the sea-grass by the white encrusting algae that coat its spines. Some extinct cidaroids had huge paddle-shaped spines that were almost certainly an adaptation to encourage the attachment of small encrusting animals and plants.
Another form of camouflage is adopted by some regular echinoids that have suckered aboral tube-feet. They cover themselves with coarse particles picked up from the sea floor and held aborally by their tube-feet. For species that live among seaweeds or sea-grasses, the tube-feet are used to wrap algal fronds or leaves around the body.
The great majority of irregular echinoids live out of sight within the sediment and thus avoid most visual predators.