The sea-dwelling phylum Echinodermata contains some of the strangest marine animals on record. We’re all familiar with starfish, and anyone who has gazed in a tide pool has seen a sea urchin. We’re perhaps less familiar with other classes of echinoderms, such as the sea lily or the brittle star. There’s a reason for this lack of familiarity: out of 7,000 known species, none of them are terrestrial! Let’s learn more about this reclusive phylum, the stars of the sea.
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PENTARADIAL SYMMETRY Adult echinoderms’ bodies exhibit radial symmetry. This symmetry is usually (but not exclusively) pentaradial, meaning that the adult echinoderm’s body has five symmetrical components arranged around a central axis. This is in contrast with their larval form, which exhibits bilateral symmetry.
WATER VASCULAR SYSTEM Echinoderms move thanks to an ingenious water vascular system. An opening known as the madreporite allows water to enter the echinoderm. This water first enters a central ring canal and is then distributed down each arm of the echinoderm into various radial canals. The hydrostatic pressure generated by this water causes tube feet on the echinoderm’s arms to expand or retract, making movement possible.
SPINY SKIN The name “echinoderm” comes from the phylum’s characteristically spiny protective endoskeleton. This endoskeleton is made up of calcium-containing ossicles. The spines of sea urchins and other echinoderms are a specialized form of ossicle. Echinoderms have additional protection in the form of pedicellariae, or minute, pincerlike organs.
ASTEROIDEA Asteroidea is perhaps the best-known class of phylum Echinodermata, containing starfish. There are nearly 2,000 extant species of asteroidea, many of which play a major role in the health of marine ecosystems. The crown-of-thorns starfish, for instance, can be devastating to coral reefs.
CRINOIDEA Crinoidea is another, smaller, class of Echinodermata. Crinoids, also known as Sea Lilies, are its best-known members. Crinoidea are filter feeders, and they exhibit an anatomical feature known as an ambulacral groove, an opening on the underside of each arm which helps move food from the mouth outwards.
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Quizbowl is about learning, not rote memorization, so we encourage you to use this as a springboard for further reading rather than as an endpoint. Here are a few things to check out:
Read this article to learn more about the evolutionary origins of the water vascular system.
What is Aristotle’s Lantern and what does it have to do with echinoderms? Find out here!
We don’t normally associate starfish with vicious predators, but some are! Check out these fierce members of the family.
Members of class Holothuroidea have a unique, albeit gross, form of self-defense. Watch more here!
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