BIO201 Echinoderms Notes

Echinodermata Overview

  • Approximate number of species: ~6,000

General Characteristics of Echinodermata

  • Successful Simplicity:

    • Lack of complex organ systems:

    • No excretory systems.

    • No significant circulatory system.

    • No centralized respiratory structures.

    • Nervous System: Very simple, distributed nervous system.

    • Innovations:

    • Mutable tissue.

    • Water vascular system for various functions.

Body Symmetry

  • Pentamerous Radial Symmetry (also known as Pentaradial Symmetry):

    • Body divided into five or more arms.

    • Orientation terms such as dorsal/ventral, left/right, anterior/posterior are not applicable.

Developmental Note

  • Larvae exhibit bilateral symmetry, which becomes radial symmetry in adulthood (secondary radial symmetry).

Body Orientation in Echinoderms

  • Example: Holothurians (sea cucumbers).

Synapomorphy: Water Vascular System

  • Definition: A hydraulic system unique to Echinodermata.

  • Functions:

    • Gas exchange.

    • Locomotion.

    • Feeding.

  • Key Structures:

    • Madreporite (entry point for water).

    • Ampulla of madreporite.

    • Stone canal.

    • Tube foot configuration:

    • Ampulla.

    • Sucker at the terminal end.

    • Ring canal and lateral canal.

    • Radial canal.

Water Vascular System Details

  • Tube feet represent the only portion of the water vascular system visibly accessible from the exterior.

Endoskeleton Composition

  • Types of Echinoderms and their endoskeleton structures:

    • Asteroids (sea stars): Loosely connected ossicles resemble "chain mail".

    • Echinoids (sea urchins): Ossicles fused to form a solid test and spines.

    • Holothurians (sea cucumbers): Ossicles remain as simple plates within the body wall.

    • The epidermis covers a mesoderm-derived dermis containing a calcareous endoskeleton.

Mutable Connective Tissues

  • Also referred to as Catch Connective Tissue:

    • Material properties of collagenous connective tissues are under neural control.

    • Consistency of materials is modulated by the surrounding osmotic balance.

    • This adaptation minimizes energy costs during sustained muscular activity.

Tube Feet and Gas Exchange

  • Functions: Tube feet play roles in gas exchange and excretion through passive diffusion.

Classification within Phylum Echinodermata

  • Class Asteroidea: Sea stars.

  • Class Echinoidea: Sea urchins and sand dollars.

  • Class Holothuroidea: Sea cucumbers.

Class Asteroidea - Sea Stars

  • Body structure:

    • Typically possesses 5 or more arms that are not distinct from the central disc.

    • Madreporite and anus are located on the aboral side of the organism.

  • Feeding mechanisms: Sea stars feed through gastric eversion.

Asexual Reproduction and Regeneration in Asteroids

  • Renowned for regenerative abilities; certain species like Linkia can regenerate an entire body from just one arm.

  • Asteroid larvae capable of asexual budding.

Class Echinoidea - Sea Urchins

  • Approximately 1,000 living species.

  • Features:

    • The skeleton is fused into a solid structure known as a test with movable spines, including both urchins and sand dollars.

  • Anatomy:

    • Primary and secondary spines present.

    • Equipped with sensory rings and mucous glands.

Class Holothuroidea - Sea Cucumbers

  • Approximately 1,100 species.

  • Anatomical features:

    • Elongated, fleshy body oriented along the oral-aboral axis.

    • Skeleton consists of isolated ossicles.

    • Feeding tentacles arranged in a circle around the mouth.

  • Interesting facts on sea cucumbers:

    • Gut defenses against predators.

    • Anal breathing behavior for respiration.