irregular echinoids

A Dictionary of Earth Sciences | 1999 | 564 words | Copyright

irregular echinoids Echinoids (Echinoidea) in which a bilateral symmetry is superimposed on the radial symmetry and where the apical system no longer contains the periproct. In some groups, e.g. the spatangoids, the aboral portions of the ambulacra (see AMBULACRUM) terminate above the ambitus and are situated in recessed, leaf-like ‘petals’, the mouth is situated far forward, and behind the mouth there is a flattened area (the ‘plastron’) that bears flat spines and is formed from the posterior interambulacra (see INTERAMBULACRUM).

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AILSA ALLABY and MICHAEL ALLABY. "irregular echinoids." A Dictionary of Earth Sciences. 1999. Encyclopedia.com. 27 May. 2012 <http://www.encyclopedia.com>.

AILSA ALLABY and MICHAEL ALLABY. "irregular echinoids." A Dictionary of Earth Sciences. 1999. Encyclopedia.com. (May 27, 2012). http://www.encyclopedia.com/doc/1O13-irregularechinoids.html

AILSA ALLABY and MICHAEL ALLABY. "irregular echinoids." A Dictionary of Earth Sciences. 1999. Retrieved May 27, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O13-irregularechinoids.html

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