Echinoderms are a group of animals that include starfish, brittlefish, sea lilies, sea daisies, sand dollars, sea cucumbers and sea urchins. The are, arguably, among the oddest animals alive today.
The most obvious of their unique characteristics is their five-sidedness. This means that their body parts are arranged around the axis of the mouth in five equal segments. But it should be noted that echinoderms begin their lives as bilaterally symmetrical larvae. During development, one section of the larva develops into the five sections of the adult form.
Echinoderms also exhibit a variety of body layouts. In some groups (such as the sea lilies and feather stars) have a mouth and anus that point upward (these animals live attached to the substrate). Other groups (such as starfish and brittle stars) have a mouth that points downward and an anus that points upward. Then there are the sea cucumbers which simply lie about on their sides. So echinoderms are extremely varied in the layout of their body parts.
Another oddity about echinoderms is that they have no brain. Instead, they have a well developed system of nerves but no distinct nerve center that could be labeled a brain.
Photo © Stephen Frink / Getty Images.
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