ECHINODERMATA. 



Radially symmetrical animals, with calcareous plates in the 

 integument. Water-vascular system always present. 



CLASS 1. Asteroidea. 



With radiating arms not sharply defined from 



the central disk. Ambulacral feet in grooves 



on the oral side. 

 Order 1. Phanerozonia. 



With large marginal ossicles. (Astropecten.) 

 Order 2. Cryptozonia. 



Marginal ossicles inconspicuous. (Asterias.) 

 CLASS 2. Ophiuroidea. 



With slender radiating arms sharply defined 



from the central disk. No ambulacral grooves. 

 Order 1. Ophiurida. 



Arms not branched. (Ophiura.) 

 Order 2. Euryalida. 



Arms branched. (Astrophyton.) 

 CLASS 3. Echinoidea. 



Globular, or somewhat disk-shaped, spiny bodies. 



Shell or test composed of close-fitting plates. 

 Order 1. Regularia. 



Nearly globular test. Spines rather large. 



Mouth and anus polar. Jaws present. (Ar- 



bacia, Strongylocentrotus.) 

 Order 2. Clypeastroidea. 



More or less flattened test. Spines very small. 



Anus not polar. Jaws present. (Echinarach- 



nius.) 

 Order 3. Spatangoidea. 



Somewhat flattened and elongated. Spines very 



small. Neither mouth nor anus polar. 

 CLASS 4. Holothuroidea. 



Bodies soft, elongated and cylindrical. Mouth 



and anus polar, the former surrounded by a cir- 

 clet of large oral tentacles. 

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