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Urchin Anatomy

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  • Hydraulic: caused by liquid under pressure.
  • Symmetry: equally balanced arrangement.

Sea Urchin Anatomy

One look at a sea urchin and you can see why they would be called sea hedgehogs. They have hard rounded shells covered with sharp movable spines. Urchins are part of the phylum Echinoderm and their name comes from Ancient Greek (echinos meaning "hedgehog" and derma meaning "skin"). There are more than 900 species of sea urchins and they come in a range of colors including purple, blue, brown, green, and red.

If you look inside a sea urchin you will see there are some organs that are familiar, like the esophagus and intestine. There are also some parts that are different than what you find in many other animals. The most unique organs are the ones that are part of the water system that acts like a hydraulic pump.

Looking Inside a Sea Urchin

Sea Urchin Anatomy

1-Genital plateThere are several plates surrounding the anus. Each plate has a single duct where the gametes (eggs or sperm) can be released into the water.
2-GonoporeThis is the opening at the end of the duct in the genital plate where eggs and sperm are released.
3-AnusNotice the anus is located at the top of the urchin while the mouth is at the bottom.
4-Hard plate with madreporiteWater enters through this space into the water vascular system of the urchin.
5-Axial glandThis gland has been investigated by biologists for some time. It has been thought to be part of the immune system and part of the circulatory system.
6-GonadOrgan that makes gametes (sperm or eggs). Sea urchin eggs have a golden color.
7-IntestineMakes digestive enzymes and further breaks down food that came from the esophagus.
8-AmpullaeWhen the ampullae contract, the tube feet stretch as water moves into them. This allows the urchin to extend their tube feet. Muscles are used to contract the tube feet and push the water back into the ampullae.
9-TestThis is the exoskeleton, which is made of calcium carbonate. Our bones also have calcium carbonate in them to give them strength.
10-Radial canalAmpullae branch off from either side of the radial canals.
11-EsophagusThe tube that runs through the center of Aristotle's Lantern and passes food to the intestine.
12-Aristotle’s lanternThe organ that chews and ingests food.
13-TeethThese teeth act like jaws. There are five teeth in most sea urchins and they are part of the organ called Aristotle's lantern.
14-MouthBelieve it or not, urchins have lips of soft tissue that also have small bony pieces embedded inside.
15-Nerve ringSea urchins can detect touch, chemicals, and light.
16-Ring canalA muscular ring that circles the eosphagus and is part of the water vascular system.
17-Test platesSections that form the test (exoskeleton).
18-Tube feetUsed for locomotion and to move loose food particles toward the mouth.
19-SpinesUsed for defense and movement. If broken they can regenerate.

 

Sea Urchin Water System

The water system of a sea urchin helps control its tube feet, which allow it to move and to grasp food particles. It works like a hydraulic system. Sea urchins aren't the only animals with a water system like this. Other echinoderms like sea stars also have a water system.

Sea Urchin Water System

1-MadreporiteA filtered opening that allows water into the water vascular system.
2-AquiferWater-filled area.
3-Ring canalThe ring canal circles the intestine and has five branches called radial canals.
4-Radial canalThere are five radial canals that supply water to the tube feet.
5-AmpullaPart of the tube foot that expands when water is forced inside and contracted by using internal muscles.
6-PodiaSuction end found at the end of each tube foot.
    


Sea Urchin Symmetry

You might not know it, but sea urchins have a fivefold symmetry just like sea stars. While the sea star's five arms make it easy to see its symmetry, it is not as obvious that the closely related sea urchin also has a fivefold symmetry.

Sea urchin fivefold symmetry

Sea urchin shell

A sea urchin shell shows its fivefold symmetry. If you look carefully you can see the pattern.

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Sea urchin shell

A sea urchin shell shows its fivefold symmetry. If you look carefully you can see the pattern.

Share to Google Classroom

Be part of Ask A Biologist

by volunteering, or simply sending us feedback on the site. Scientists, teachers, writers, illustrators, and translators are all important to the program. If you are interested in helping with the website we have a Volunteer page to get the process started.