Phylum Echinodermata

Welcome to the World of Echinoderms

Echinoderms have about 7,000 species and are exclusively marine.  Adult echinoderms have radial symmetry.  They are characterized by there spiny skin, which is where they got their name!  In Greek, "echinos" means spiny and "dermos" means skin.  Examples of echinoderms include sea stars, sea urchins, sea cucumbers, sand dollars, and brittle stars.

Types of Echinoderms

Patiria miniata

Sea Stars (starfishes) are one of the most recognized echinoderms.  There are approximately 1,800 species of sea stars.  They have thick arms (ambulacra), two stomachs, and can produce digestive enzymes to put on prey, even before they eat it!

 Echinothrix calamaris

Sea Urchins are characterized by their spiny, spherical shapes.  There are around 950 species of sea urchins.  Along their test (internal skeleton), are long spines and pedicellariae (pincer-like organs); these can be poisonous.  Another poisonous component of sea urchins are their teeth.  They have a complicated dental structure call Aristotle's Lantern.  The teeth are very strong, and sharp enough to cut through steel!

Crinoidea

Sea Lilies are different from other echinoderms because they are sessile as adults.  They are attached to the ocean floor by stalks, and resemble plants with frond-like tentacles.  There are only about 80 species of sea lilies.

Water Vascular System

Aboral Side of Sea Star

One thing that echinoderms are known for, are their water vascular systems.  They use this system to help feed, respire, and move.  Before learning about the water vascular system, it's helpful to know the main anatomy of echinoderms.  For sea stars, they have oral and aboral sides.  The oral side is where the mouth is, and you can see its many tube feet.  The aboral side is where the anus and sieve plate (madreporite) openings are.  The arms of the sea star are called rays.  The circular section where the rays attach is called the central disk.  

Water Vascular System of Sea Stars

Water Vascular System of Sea Urchins

Now, let's learn the stages of the water vascular system.  First, water enters through the sieve plate (madreporite).  After that, it goes down the stone canal and enters the ring canal.  The ring canal is circular and runs through the central disk.  Next, it travels to the lateral/radial canals, which go through each ray.  Finally, it enters the ampullae (singular: ampulla) which is part of the tube feet.  It is stored there until muscles contract the water in or out.  When it's contracted, pressure is created, which makes the tube feet act like a suction cup.  This allows the echinoderms to move. 

Echinoderms and Medicine

Bohadschia argus

Researchers continue to look for biological compounds in echinoderms that can be used for medicinal applications.  The Chinese have already been using sea cucumbers to treat a variety of ailments for hundreds of years.  Echinoderms have been found to have compounds that are can slow down the rate of tumors.  So, they have been very important in cancer research.  One interesting fact about echinoderms is their ability to regrow parts of their bodies.  This has made echinoderms become a part of the research needed to figure out how to grow human body parts.  Also, many echinoderm eggs are transparent, which makes them helpful for embryo development research.