Image Key > Echinodermata > Echinoidea

• Less than 1 mm.

• Sea urchins; the small and spiny creatures found in the oceans of the world.
• Planktonic larva is called pluteus larva.

• Bilateral symmetry.
• Transparent.
• Calcareous skeleton and spines.
• Spines simple or fenestrated.
• Skeleton is birefringent (the decomposition of a ray of light, passing through the skeleton and being split into two rays). Skeleton becomes visible in glowing rainbow colours that change as the swimming larva changes its orientation to the light).

• Common worldwide.

• Separate sexes that spawn via broadcasting.
• Females may release millions of eggs.
• Swim with arms upward, resembling a pyramidal prism.
• Locomotion is via a current created by cilia that run in a band over body of each arm.
• Pluteus larvae live in the plankton for weeks to several months.
• Non-feeding larvae have a much shorter planktonic period.