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EVOLUTION OF ANIMALS 



Part V 



CLASS CRINOIDEA 

 Sea Lily 



CLASS OPHIUROIDEA 

 Brittle Star 



CLASS HOLOTHUROIOEA 

 Sea Cucumber 



CLASS ASTEROIDEA 

 Sea Stor 



CLASS ECHINOIDEA 

 Sand Dollar 



Fig. 32.1. Echinoderms. Animals of this group are built upon a plan of five. 

 Details of the plan vary greatly, but five and multiples of five appear as insistent 

 and clear as the theme of a symphony. Of those shown in this figure, the five is 

 least apparent in the sea urchin and sea cucumber. If the sea urchin were moving 

 however, its five rows of tube-feet would reach out to the rock; and if the sea 

 cucumber were active five pairs of respiratory gills would be expanded in the water. 

 The starfish has so stretched its stomach into the clam that the five lobes are pulled 

 out of their regular shape during this meal time. 



numerous on the internal organs as well as on the outer surfaces of the body, 

 the latter a proof of complete limitation to life in the water. With the rare 

 exceptions of certain feather stars, only the larvae can swim. Nearly all 

 echinoderms live on the bottom and minute organisms and particles are always 



