Phylum Echinodermata: Definition, Characteristics & Examples

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  • 0:02 What is an Echinoderm?
  • 1:07 Examples of Echinoderms
  • 2:45 Defining Features
  • 5:14 Lesson Summary
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Lesson Transcript
Instructor: Heather Pier

Heather has taught high school and college science courses, and has a master's degree in geography-climatology.

Learn about echinoderms, which include starfish and sand dollars, in this lesson. We'll discuss what an echinoderm is, examine what their defining features are, and go over some examples of the five classes that make up the phylum Echinodermata.

What Is an Echinoderm?

The name may sound strange, but I can assure you that echinoderms will be anything but foreign to you, especially if you frequent the beach, the aquarium, or tide pools. Some of the most classic examples of marine invertebrates, including starfish, sand dollars, and sea urchins, are all echinoderms, which are part of the phylum known as Echinodermata. The name Echinodermata comes from the Greek word for 'spiny skin,' and if you look closely at a member of the phylum, you'll see that the name is more than appropriate.

In one form or another, all echinoderms are covered in some form of bumps (starfish), spikes (sea cucumbers), or spines (sea urchins). Even sand dollars are covered in bumps when they are alive, though these features aren't as prominent in the dry, preserved form of sand dollar that most people encounter. Echinodermata is a phylum of many unique and beautiful sea creatures which all share a set of common defining traits.

Examples of Echinoderms

Unlike other invertebrate phyla, which contain hundreds or thousands of different types of animals, Echinodermata only contains five different classes of animals:

  • Starfish, or sea stars, are the classic echinoderms that most people think of. They have five arms of varying thickness and can come in a variety of colors and textures.
  • Brittle stars, sometimes called serpent stars, look like typical starfish except their legs are much skinnier and more fragile (hence the 'brittle' in their name), and they have more prominent spines than the sea stars.
  • The technical name for sand dollars, sea urchins, and heart urchins is echinoid. Unlike the sea stars and brittle stars, echinoids contain no arm-like appendages. Instead, they are shaped like globes or half globes. They are rounded on the top and flat on the bottom and feature spines of varying length, with sea urchins having longer spines and sand dollars and heart urchins having shorter spines.
  • Sea cucumbers look similar to their namesake, the vegetable cucumber. They have long, cucumber-like bodies with bumps on the surface.
  • The most ancient of the echinoderms, the sea lilies, are the lone sessile, or immobile, organisms in the phylum. Fossils of sea lilies are not unusual in regions that used to be located near oceans. All sea lilies have at least ten appendages extending from the main stalk of their body, which they use to feed on detritus, or organic matter floating through the ocean.

Defining Features

All echinoderms are marine species, meaning they reside in saltwater and only saltwater; there are no terrestrial or freshwater echinoderms. The bodies of echinoderms are primarily made of calcium plates or shell-like structures, which, depending on the organism, can be tightly or loosely held together. Sand dollars have a much tighter set of plates, whereas the more mobile sea urchins and sea cucumbers have more loosely held plates.

All echinoderms are also bottom dwelling, meaning that even though they live in the ocean, they aren't able to swim. They are, however, mobile, meaning they can move about on the ocean bottom; they simply can't swim! All echinoderms will always be on the ocean floor, making them easy to locate if you go swimming or diving because you always know where to look for them. Just be sure not to step on one, especially the spiny sea urchins! Many of the spiny varieties of echinoderms have poisonous spines for self-defense, but these spines can also puncture or tear the skin, so it's important to be careful around most members of the phylum.

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