Asterozoans (Class Stelleroidea) are grouped into three subclasses;
The first true asteroids are of early Arenig
This subclass contains the asteroids commonly known as star fish. However due to the fact they are not fish, most biologists encourage the use of the term sea stars.
There are two orders;
Ophiurida (Ordovician - Recent) - the arms only bend in the horizontal plane
Euryalae (Carboniferous to Recent) - the arms can move in all directions, and are often climbers.
Also known as brittle stars these asterozoans are instantly identifiable by their thin snake-like arms, shown in the above picture. These have contributed to their great success, as they use sinuous movements of their arms to move.
Tremadocian - Middle Ordovician
The earliest known sea stars belong to this subclass, which was a short-lived group that was probably the common ancestor to both Asteroidea and Ophiuroidea.
continue to learn more about the anatomy of sea stars, or
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Author: Elizabeth Sweet
Last updated: 22nd November 2005
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