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Echinodermata (1)

Phylum Echinodermata

Overview

  • Echinoderms include sea stars, sea cucumbers, and sea urchins.

  • Characterized as a noble group uniquely complex, leading to intrigue within zoology.

  • Free-living marine species, approximately 7,000 identified.

  • Notable for high regeneration capacity.

Developmental Characteristics

Key Features of Protostomes vs. Deuterostomes

  • Protostomia and Deuterostomia are subdivisions of Bilateria.

  • Echinoderms fall under Deuterostomia:

    • Blastopore Development: First forms anus, mouth forms secondarily.

    • Cleavage Patterns:

      • Deuterostomia: Radial cleavage.

      • Protostomia: Spiral cleavage.

  • Coelom Formation:

    • Deuterostomes form coelom via outpocketing (enterocoelous).

    • Protostomes form coelom via splitting (schizocoelous).

Key Traits of Echinodermata

  • Symmetry: Primarily exhibit pentaradial symmetry as adults, with no distinct cephalization.

  • Body Plan:

    • Distinct oral (mouth) and aboral (anus) sides.

    • Absence of specialized excretory or circulatory systems, relying on diffusion for gas exchange.

Water Vascular System

  • Synapomorphy of Echinoderms:

    • Water-vascular system: A network of fluid-filled tubes and ampullae.

    • Utilized primarily for locomotion through tube feet and hydraulic function.

Morphological Features

Endoskeleton

  • Composed of ossicles: Calcareous plates form a rigid structure.

  • Internal skeletal components include:

    • Papulae: For gas exchange.

    • Various canals and muscles facilitating movement and feeding strategies.

Larval Stages

  • Bilateral larvae: Initially exhibit bilateral symmetry before maturation into adult forms.

  • Metamorphosis: Transition through larval stages like Bipinnaria and Brachiolaria leading to adult forms.

Reproductive Strategies

Sexual Reproduction

  • External fertilization with a complex indirect life cycle.

  • Development of bilaterally symmetrical larvae from fertilized eggs.

Asexual Reproduction

  • Capable of fission and fragmentation, enhancing survival strategies among species.

  • Notably, sea stars can regenerate limbs after splitting across the disc.

Classifications within Echinodermata

Major Classes

  1. Class Asteroidea (Sea Stars)

    • Most organs located in the central disk.

    • Characterized by 5 broad arms extending from the disk.

    • Feeding structure involves a complex stomach mechanism.

  2. Class Ophiuroidea (Brittle and Basket Stars)

    • Features 5 narrow arms attached to a central disk.

    • Lack of central organs in arms, typically deposit feeders.

  3. Class Echinoidea (Sea Urchins)

    • Characterized by a test (shell) formed by fused ossicles.

    • Primarily algal grazers with slow locomotion using tube feet and/or spines.

  4. Class Holothuroidea (Sea Cucumbers)

    • Possess no arms but exhibit secondary bilateral symmetry in some.

    • Utilization of tube feet for feeding and locomotion with reduced ossicles.

    • Possess unique defense mechanisms including toxins and evisceration.

Evolutionary Significance

  • Echinoderms have evolved unique structures and systems diverging from ancestral bilateral forms, leading to their distinct ecological roles.

  • Their adaptations allow for a variety of feeding strategies and survival mechanisms in marine environments, underscoring their evolutionary success.