The Phylum Echinodermata includes familiar marine invertebrates like sea stars, sea urchins, and sea cucumbers, presenting one of the most intriguing body plans in the animal kingdom. These organisms, whose name literally means “spiny skin,” are exclusively found in marine environments. Symmetry is a fundamental concept in biology, describing the balanced distribution of an organism’s body parts. Their unique anatomical arrangement highlights a fascinating evolutionary history.
Defining Echinoderms and Radial Symmetry
Echinoderms are defined by their unique hydraulic network, known as the water vascular system, which is used for locomotion, feeding, and gas exchange. This complex system opens to the exterior through a sieve-like plate called the madreporite. Common examples of this diverse group include the five-armed sea stars, the globular sea urchins, the elongated sea cucumbers, and the delicate brittle stars.
Radial symmetry describes an organism whose body parts are arranged around a central axis, much like the spokes of a wheel. An animal with this symmetry has a distinct top and bottom, or oral and aboral sides, but lacks a head or any true left and right sides. This body organization contrasts sharply with bilateral symmetry, which involves a single plane dividing the organism into mirror-image halves. Radial symmetry is advantageous for organisms that are sessile or move slowly, allowing them to interact with their environment equally from all directions.
The Adult Form: Pentaradial Arrangement
Adult echinoderms exhibit a distinctive modification of radial symmetry known as pentaradial symmetry, where the body is structured around five segments or multiples of five. This five-part arrangement is a hallmark feature present across all living classes of the phylum. For example, the five arms radiating from the central disk of a sea star or a brittle star clearly display this pattern.
In a sea urchin, the pentaradial layout is visible through the five distinct rows of tube feet and the arrangement of its skeletal plates, which fuse to form a rigid internal shell called a test. Even the worm-like sea cucumbers, which appear superficially bilateral, reveal pentaradial symmetry internally through their five ambulacral areas. This body plan is functional for their bottom-dwelling lifestyle, enabling them to sense prey, respond to stimuli, and move slowly without a primary direction of travel.
The Developmental Shift from Bilateral Larvae
The adult radial body plan is a secondary development, as the larval stages of echinoderms are initially bilaterally symmetrical, like most other complex animals. These tiny, free-swimming larvae, such as the bipinnaria or the pluteus, possess clear left and right sides and are ciliated for movement through the water column. The presence of this bilateral stage strongly suggests that the ancestors of echinoderms were also bilateral, positioning them evolutionarily closer to vertebrates than to simpler radial animals like jellyfish.
As the larva prepares to settle and transform into a juvenile, it undergoes a profound metamorphosis. During this process, the bilateral larval structure is almost entirely reorganized and absorbed, with the adult body developing from a small region on the larval left side. The original bilateral form is discarded, and the new five-part radial body plan emerges. This developmental shift illustrates that the radial symmetry of the adult is a specialized adaptation rather than a primitive trait.