Taormina has taught advanced high school biology, is a science museum educator, and has a Master's degree in museum paleontology.
Echinoderm Structure: Skeleton & Symmetry
What Is An Echinoderm?
First of all - how do you say it? The Phylum Echinodermata (ekkin-o-der-MAH-tah) might be a tongue twister, but you should be familiar with this group of animals. They're our cousins!
Echinoderms are members of the superphylum Deuterostomia; we humans, and all other chordate animals, are also a part of this superphylum.
In deuterostome embryological development, the anus forms first and the mouth forms second. That's where the name comes from: the Greek deutero-, or second, and the Greek -stomia, or mouth.
Echinoderms include sea stars or starfish, sea lilies, sea cucumbers, sea urchins, and sand dollars. The name echinoderm derives from the Greek words for 'hedgehog' - yes, the spiny animal! - echinos, and the word for 'skin', dermos. So, echinoderms have spiny skin.
All echinoderms have some form of radial symmetry as adults, meaning they have body parts that branch out from a central point - think of the five arms of a starfish. That's fivefold radial symmetry.
Echinoderms are also united by a water vascular system that pushes water throughout their bodies, and by tube feet with tiny suckers on the ends. The Phylum Echinodermata doesn't always have spiny skin, or a spiny surface, but these animals do all share a calcium carbonate internal skeleton, or endoskeleton.
Let's talk a bit more about how the skeletal structure and symmetry of echinoderms works.
Echinoderm Skeleton Structure
![]() |
Think of an ornate building with windows and bricks. It has interlocking blocks that form beautiful spires and towers, with windows of different shapes and sizes.
The endoskeleton of echinoderms is built just like an ornate and beautiful building. Just like a building has windows within its connected blocks, the internal calcium carbonate skeleton of echinoderms is made up of connected plates.
There are tiny pores, and openings for the mouth and anus in these plates, much like the windows in a building. And, because the skeleton is covered in an epidermis, or outer skin covering, it is internal - that's where the endo- in endoskeleton comes in, since it means inside in Greek.
Some echinoderms have very tightly interlocking plates, like sea urchins. Think of them as tight defense systems around an armed building. In sea urchins and sand dollars, these interlocking plates form an enclosure, almost like a shell, called a test. Sea urchins also have spines protruding from their tests, in life.
Others have more movable plates, like sea stars or starfish. Sea cucumbers, the worm-like echinoderms, have an endoskeleton that is nearly microscopic. They may not have as strong a defense system, but they are much more flexible and movable.
If you zoom in on an echinoderm's calcium carbonate skeletal plates under an electron microscope, you'll see that each plate is made up of a very fine webbing. Each piece of webbing is actually a crystal of calcium carbonate that has branched and spread to form an overall skeletal network, called a stereom.
Echinoderm Symmetry
Since we've been discussing ornate buildings, let's think about how a building might be organized. Imagine a structure like the Pentagon building in Washington, D.C., with corners set at five points, like a star.
This is exactly how echinoderm symmetry is arranged. It's a type of radial symmetry, which is when an animal's body plan is arranged with one central point and a branching series of body parts. The cnidarians (nid-AIR-ee-ans), or jellyfish and hydras, have radial symmetry, along with our echinoderms.
![]() |
However, echinoderms have an even more specific type of radial symmetry called pentaradial symmetry. Just like the Greek word for 'five', penta-, echinoderms have a fivefold symmetry. Your five-pointed starfish is a good example of this kind of symmetry.
You can also see pentaradial symmetry in the tentacled mouths and internal calcium carbonate skeleton of sea cucumbers, and in the tests of sea urchins and sand dollars. Even sea lilies, or crinoids, have a pentaradial arrangement in their feathery tentacles.
Even though adult echinoderms exhibit pentaradial symmetry, it's important to remember that the larvae, or young, have bilateral symmetry - symmetry that is nearly equal on either side of a midline. Humans have bilateral symmetry; imagine looking in the mirror and drawing an imaginary line down the center of your face. With the exception of a few freckles or other marks, you'd have the same features on both sides of your face. That same imaginary line can be drawn down your entire body, with the same results.
The free-floating larvae of echinoderms begin to develop pentaradial symmetry as they mature, but this initial bilateral nature unites us, as humans, with the echinoderms under the superphylum Deuterostomia.
Lesson Summary
Strange, living defense structures. Spiny, radial creatures. They aren't science fiction, they're echinoderms - and they're even our relatives!
The animals within Phylum Echinodermata are members of the superphylum Deuterostomia, along with the Phylum Chordata - the chordates. We humans are chordates, and we all share a common embryologic development within the deuterostomes. The anus develops first, rather than the mouth. We also share bilateral symmetry during our development.
The echinoderm skeleton, however, is quite different from our own skeleton. Although we share an internal endoskeleton, underneath our outer skin or epidermis, the endoskeleton of echinoderms is made up of interlocking or loosely-held calcium carbonate plates. There are pores within these plates, as well as openings for the mouth and anus. Some echinoderms, like sea urchins and sand dollars, have a very tightly interlocking series of plates known as a test.
Others have more fluid plate arrangements, such as sea stars and sea cucumbers. Even sea lilies, or crinoids, have this calcium carbonate skeleton. In microscopic view these plates make up a network of extended calcium carbonate crystals that form a structure known as a stereom.
All echinoderms also exhibit radial symmetry in their body plans - specifically pentaradial symmetry, or fivefold symmetry, which is unique among animals. Only cnidarians, like jellyfish and hydra, have anything close to this kind of arrangement.
To unlock this lesson you must be a Study.com Member.
Create your account
Register to view this lesson
Unlock Your Education
See for yourself why 30 million people use Study.com
Become a Study.com member and start learning now.
Become a MemberAlready a member? Log In
BackEchinoderm Structure: Skeleton & Symmetry
Related Study Materials
- TExES Science of Teaching Reading (293): Practice & Study Guide
- Next Gen NCLEX-PN Study Guide & Practice
- Next Gen NCLEX-RN Study Guide & Practice
- TExES Core Subjects EC-6 (391): Practice & Study Guide
- TExES School Counselor (252): Practice & Study Guide
- Multithreading, Networking & Machine Learning in Python
- Digital Forensics
- The Internet of Things and Networking
- Network Routing and Switching
- Virtual Servers and Networking
- How to Pick Your Homeschool Curriculum
- Role of Student Support in Open & Distance Learning
- TExES Principal Exam Redesign (068 vs. 268)
- Teacher Salary by State
- ESL Resource Guide for Teachers
- What is a Homeschool Co-op?
- How to Start Homeschooling Your Children
Latest Courses
- Heritability Coefficient
- Major Health Problems in the United States
- Personality Disorders: Borderline, Histrionic, Narcissistic, Schizoid & Antisocial
- Memory Hole in 1984
- Twelfth Night Act 1 Scene 4: Summary & Analysis
- Commercial Speech: Definition & Examples
- Legal Moralism: Definition & Examples
- Quiz & Worksheet - Comparing Social Darwinism & Darwinism
- Saint Peter: Quiz & Worksheet for Kids
- Quiz & Worksheet - Fahrenheit 451
- Finding a Character's Emotions by Actions: Quiz & Worksheet for Kids
- Flashcards - Real Estate Marketing Basics
- Flashcards - Promotional Marketing in Real Estate
- Middle School Math Worksheets
- Argumentative and Persuasive Essay Topics for Teachers
Latest Lessons
- Human Growth and Development: Tutoring Solution
- College Composition for Teachers: Professional Development
- Environment & Humanity for Teachers: Professional Development
- AEPA Business Education (NT309): Practice & Study Guide
- GACE History (534): Practice & Study Guide
- International Business Law Lesson Plans
- AP Environmental Science - Food and Agricultural Resources: Tutoring Solution
- Quiz & Worksheet - Circular Flow Model
- Quiz & Worksheet - Types of Compensation Systems
- Quiz & Worksheet - Mandatory vs. Voluntary Employee Benefits
- Quiz & Worksheet - WTO History & Role
- Quiz & Worksheet - Pros & Cons of Performance Appraisals
Popular Courses
- Using Networking Diagrams for Task Breakdown & Tracking
- Sue in The Last Leaf
- Inference Lesson Plan
- Praxis Tests in Nebraska
- CBEST Test Cost
- 504 Plans in Maryland
- How to Study for the HSPT
- What are Passing Scores for the Praxis Tests?
- IELTS General Training Writing: Format & Task Types
- ABC Games for Kids
- Average AFQT Scores
- What is the ASVAB Test?
Popular Lessons
Math
Social Sciences
Science
Business
Humanities
Education
History
Art and Design
Tech and Engineering
- Tech and Engineering - Videos
- Tech and Engineering - Quizzes
- Tech and Engineering - Questions & Answers