How do echinoderm eggs develop into larvae and juvenile animals?

Echinoderms exhibit two primary developmental pathways: indirect and direct development.

Indirect Development

In indirect development, fertilized eggs typically develop into free-swimming, planktonic larvae. These larvae begin as a blastula and progress to a gastrula, eventually forming a dipleurula-type larva characterized by bilateral symmetry. Depending on the species, these larvae may feed on small organisms or subsist on their own yolk.1

Specific larval forms include:
* Auricularia: Found in holothurians.1
* Bipinnaria and Brachiolaria: Stages seen in asteroids (starfishes).1
* Pluteus: Complex larvae with calcite skeletal rods, found in echinoids and ophiuroids.1
* Doliolaria: A barrel-shaped larva found in crinoids and some holothurians.1

After a period in the plankton, larvae undergo metamorphosis, where their bilateral symmetry is replaced by radial symmetry, and they transition into juvenile forms on the seafloor.1

Direct Development

In direct development, large, yolk-rich eggs bypass the larval stage entirely, transforming directly into juveniles. This process often involves parental care, where the female retains the young on her body surface or within specialized pouches to protect them until they are born as juveniles.1