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EVOLUTION OF ANIMALS 



Part V 



(Asterias). There are certain species that feed on small snails, taking them 

 into the stomach in the regular way. After the soft parts are digested these 

 starfishes spit out the shells, following the custom that has persisted from 

 ameba to man. An intestine and anal opening are practically nonfunctional. 



Excretion. Many ameboid blood cells are drawn into the skin gills by the 

 cilia which line them. Such phagocytic cells, usually carrying waste matter, 

 gradually work their way through the thin membranes of the gills into the open 



Fig. 32.9. Brittle-stars (Ophiothrix fragilis), the most agile of the echinoderms. 

 They are named for their ability to snap off their arms. This species is common 

 in Great Britain; others with similar habits live on rocky coasts of North America. 

 They are usually wedged in between rocks, tangled with seaweeds or one another. 

 When scattered on the bottom of a large aquarium without rocks or seaweed 

 brittle-stars will clump together within ten minutes and twine their arms about 

 one another. This and others of his experiments with brittle-stars are mentioned 

 by W. C. Allee in his book The Social Life of Animals. (Photograph courtesy, 

 Douglas P. Wilson, Marine Biological Laboratory, Plymouth, England.) 



