Echinodermata represent the most complicated metamorphosis in the course of development.
Larva have bilaterally symmetry and have a free-swimming, planktonic existence.
Adults have radial symmetry.
Class I: Asteroidea
Bipinnaria Larva
After hatching, the larva develops cilia and has a free swimming life.
Median arm + 5 pairs of ciliated arms which do not have any skeletal support inside.
Arms: pre-oral, anterior-dorsal, posterior-dorsal, posterior lateral, post-oral.
Larval gut is divided into mouth, oesophagus, stomach and intestine.
Pre-oral and post-oral ciliary bands.
After free swimming existence for a few weeks, it changes into Brachiolaria larva.
Seen in Starfish.
Brachiolaria Larva
Formed after 6-7 weeks of life and growth of bipinnaria.
Sedentary and remains attached to a hard substratum.
Possesses three brachiolarian arms having adhesive discs at the tip.
Shows all the arms that are seen in the Bipinnaria, but these arms are very long and hanging.
Ciliated arms get reduced and become thin and functionless.
Mouth, anus and gut are well developed.
Axocoel, hydrocoel and somatocoel that later on give rise to water vascular system.
Development of starfish takes place inside the sedentary brachiolaria.
Ruptures and releases tiny starfish into water.
Class II: Ophiuroidea
Ophiopluteus Larva
Free swimming larva in brittle stars.
Posterior-lateral arms are the longest and form earliest.
Post Oral, Anterior-lateral and Posterior-dorsal develop after that.
Ciliated bands on arm edges.
Internally, the larva contains coelomic chambers and archenteron.
Class III: Echinoidea
Echinopluteus Larva
Formed after gastrulation.
Stomodaeal invagination communicates with Archenteron.
Gut is differentiated into mouth, oesophagus, stomach and intestine.
Blastopore as larval anus.
6 pairs of arms: Pre-oral, Antero-lateral, Antero-dorsal, Post-Oral, Postero-dorsal, postero-lateral.
Post lateral arms are very short. Antero-dorsal arms may not develop.
Tips of arms are supported by calcareous rods.
Epaulettes: thickened ciliary bands that facilitate locomotion.
Hydrocoel and Vestibule form the oral side.
Metamorphosis is extremely rapid, no attachment stage.
Class IV: Holothuroidea
Auricularia Larva
Swims about by a ciliated band which forms pre-oral and anal loop.
Larva has a curved gut with sacciform stomach, hydrocoel and right and left somatocoels.
Class V: Crinoidea
Doliolaria Larva
Body has 4-5 ciliated bands.
Apical sensory plate at the anterior end, with a bunch of cilia.
Skeleton develops.
After differentiation into prospective organs, larva attaches itself.
Internal organs rotate from ventral to posterior (90ยบ) after attachment.
Significance of Echinoderm Larva
Common origin of classes:
Except Crinoidea, which becomes sedentary.
Same general fundamental plan with bilateral symmetry.
Flattened body, looped ciliated bands, gut and enterocoelic coelom.
Probably originate from a common ancestor which was a coelomate, bilateral and free swimming.
Taxonomic Affinities:
Larval similarities do not indicate taxonomic affinities.
On the basis of larval similarities, Ophiuroids and Echinoids should be clade, and Asteroids and Holothurians should be close.
But based on paleontological and morphological evidence, asteroids and ophiuroids are closely
related. Echinoids have a different line of evolution.
Phylogenetic Affinities:
Similarities of Ophiopluteus and Echinopluteus due to convergent evolution.
Differences between closely related groups in larval form indicates divergent larval evolution (Asteroids and Ophiuroids).
Relationship with Chordates:
Auricularia larva and Balanoglossus larva show similarities.
Cleavage is indeterminate, mesoderm and coelom have similar origin in echinoderms and lower chordates.
Aid in Dispersal and Feeding:
Larvae are the main dispersive phase.
Adults are sluggish.