On the beach
Intertidal area only
Both intertidal area and subtidal areas
Subtidal area only
Pentamerous
Dome
Porcupine
Toothpick
Echinoderms
Animalia
Purpuratus
Strongylocentrotus
Other sea urchins
Sea cucumbers
Small fish
Algae
Because sea urchins have large numbers of eggs
Because a sea urchin's egg is relatively big
Because sea urchins have large numbers of relatively big eggs
Because the eggs make great omlettes
Spine
Mouth
Feet
Nose