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Spiny, Symmetrical and Strange … what am I?

Posted on: June 17, 2012

Brittle Star

My mouth and tubed feet are on my underside, my anus is on my upper side and my gonads are in my ‘armpits’! … what am I?

I am, of course, a sea star!

Sea stars belong to the phylum of animals known as echinoderms.  Echinoderms, as the name suggests, are spiny skinned animals – ‘echino’ meaning spiny and ‘derm’ meaning skin. 

Related to sea stars, are feather stars, brittle stars, sea urchins and sea cucumbers.  What do these all have in common?  Apart from being spiny skinned, which is due to their skeleton consisting of tiny calcified plates and spines, they all exhibit radial symmetry – that is, their body shapes radiate out from the centre.

Have you seen our Echinoderms Poster?  There’s a great cross sectional diagram of a sea urchin on it.  Provide this diagram to your students and ask them to research the body parts they are unfamiliar with, then engage students in a discussion about how the location of the various body parts may be adaptations to where they live, how they eat, and most importantly, how they don’t become prey themselves.

Read the article Spiny, Strange and Symmetrical: The Weird World of Echinoderms to learn more about the relatives of sea stars and sea urchins.  Ask students to complete some research to discover why sea stars, feather stars, brittle stars, sea urchins and sea cucumbers are each in a class of their own. 

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