Model Answer
0 min readIntroduction
Echinoderms, a phylum of exclusively marine invertebrates, exhibit a unique developmental pathway markedly different from other deuterostomes. Their larvae are bilaterally symmetrical, contrasting with the pentaradial symmetry of the adults. This developmental peculiarity has long fascinated biologists and provides crucial insights into the evolutionary relationships between echinoderms and other deuterostomes, particularly chordates. The larval forms – bipinnaria, brachiolaria, and pluteus – represent stages in a complex life cycle and offer valuable clues to understanding the evolutionary history of this ancient phylum. Understanding these larval stages is crucial for tracing the evolutionary lineage of echinoderms and their connection to other deuterostomes.
Echinoderm Larval Forms: Detailed Description and Labelled Diagrams
Echinoderm larvae undergo a complex metamorphosis from a bilaterally symmetrical larva to a radially symmetrical adult. The three primary larval forms are bipinnaria, brachiolaria, and pluteus, each characteristic of different echinoderm classes.
1. Bipinnaria Larva (e.g., Starfish)
The bipinnaria larva is the earliest free-swimming larval stage found in starfish (Asteroidea) and some ophiuroids. It is characterized by a pear-shaped body with bilateral symmetry.
Key Features:
- Ciliated bands: Prominent ciliated bands used for locomotion and food capture.
- Gut: A simple gut extending from the mouth to the anus.
- Hydrocoel: A fluid-filled cavity within the body.
- Skeletal spicules: Small skeletal elements begin to develop.
2. Brachiolaria Larva (e.g., Brittle Stars)
The brachiolaria larva is a modified bipinnaria larva found in ophiuroids (brittle stars and basket stars). It develops adhesive arms, called brachiole arms, which aid in attachment to the substrate.
Key Features:
- Brachiole arms: Three adhesive arms extending from the body, used for attachment.
- Similar to bipinnaria: Retains the ciliated bands and gut structure of the bipinnaria larva.
- Enhanced settlement: Brachiole arms facilitate settlement and metamorphosis.
3. Pluteus Larva (e.g., Sea Urchins, Sea Cucumbers)
The pluteus larva is the characteristic larval form of echinoids (sea urchins and sand dollars) and holothurians (sea cucumbers). It is more complex than the bipinnaria and brachiolaria larvae.
Key Features:
- Postoral arms: Five pairs of arms extending from the posterior end of the body.
- Water vascular system: Rudimentary water vascular system develops.
- Skeletal rods: Skeletal rods provide support and rigidity.
- Ciliated bands: Ciliated bands for locomotion and feeding.
Evolutionary Importance of Echinoderm Larval Forms
The larval forms of echinoderms hold significant evolutionary importance, providing insights into their phylogenetic relationships and the evolution of deuterostome development.
- Deuterostome Ancestry: The bilateral symmetry of echinoderm larvae suggests a deuterostome ancestor. Deuterostomes, including echinoderms and chordates, share key developmental features like radial cleavage and enterocoelous coelom formation.
- Relationship to Chordates: The pluteus larva, in particular, exhibits similarities to the tornaria larva of hemichordates, which are considered close relatives of chordates. These similarities suggest a possible evolutionary link between echinoderms, hemichordates, and chordates.
- Phylogenetic Reconstruction: Variations in larval morphology across different echinoderm classes have been used to reconstruct their phylogenetic relationships. For example, the presence of brachiole arms in ophiuroids supports their close relationship with asteroids.
- Paedomorphosis: Some researchers propose that the adult echinoderm body plan evolved through paedomorphosis, where larval features are retained in the adult form. The pentaradial symmetry of adult echinoderms may have originated from the bilateral symmetry of their ancestors.
The study of echinoderm larval development continues to provide valuable insights into the evolution of deuterostomes and the origins of key developmental features.
Conclusion
In conclusion, the larval forms of echinoderms – bipinnaria, brachiolaria, and pluteus – are not merely transitional stages in their life cycle but also crucial windows into their evolutionary history. Their bilateral symmetry, unique morphological features, and similarities to larvae of other deuterostomes provide compelling evidence for their phylogenetic relationships and the evolution of deuterostome development. Continued research on echinoderm larval development promises to further illuminate the evolutionary origins of this fascinating phylum and its connection to the broader tree of life.
Answer Length
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