Phylum Echinodermata — Definition

Definition

Imagine a creature from the ocean that looks like it's wearing armor made of tiny, spiky plates, and moves around using hundreds of tiny, suction-cup-like feet. That's essentially an echinoderm! The name 'Echinodermata' literally means 'spiny-skinned,' which perfectly describes many members of this phylum, such as starfish and sea urchins.

These fascinating animals are found exclusively in marine environments, meaning you'll only ever find them in saltwater, from shallow coastal areas to the deepest parts of the ocean.

One of the most striking features of adult echinoderms is their radial symmetry, usually in multiples of five (pentamerous radial symmetry). This means their body parts are arranged around a central axis, much like the spokes of a wheel.

Think of a starfish with its five arms radiating from a central disc. However, don't let this fool you into thinking they're simple organisms; their larval stages are actually bilaterally symmetrical, which is a key piece of evidence suggesting their evolutionary ancestors were bilaterally symmetrical, just like us!

This shift in symmetry during development is quite unique.

Echinoderms are also characterized by an internal skeleton, called an endoskeleton, made up of calcareous (calcium carbonate) plates or ossicles. These ossicles can be fused together to form a rigid shell, as seen in sea urchins, or remain more flexible, allowing for movement, as in starfish. Protruding from this spiny skin are often tiny pincer-like structures called pedicellariae, which help keep the body surface clean and can even aid in defense or food capture.

But perhaps the most incredible and defining characteristic of echinoderms is their water vascular system, also known as the ambulacral system. This is a complex network of water-filled canals and tube feet (podia) that extends throughout their body.

Seawater enters this system through a sieve-like plate called the madreporite. The pressure changes within this system allow the echinoderm to extend and retract its tube feet, which are used for locomotion, attaching to surfaces, and even manipulating food.

It's a truly ingenious hydraulic system!

They are triploblastic, meaning their bodies develop from three germ layers (ectoderm, mesoderm, and endoderm), and they possess a true coelom (a body cavity lined by mesoderm), which houses their internal organs.

While they lack a distinct head, brain, or specialized excretory organs, their nervous system is typically a nerve ring with radial nerves. Reproduction is usually sexual, with separate sexes, and external fertilization.

Many also exhibit remarkable powers of regeneration, capable of regrowing lost arms or even an entire body from a single arm and a portion of the central disc. This phylum includes familiar examples like sea stars (starfish), sea urchins, sea cucumbers, brittle stars, and sea lilies.