CHAPTER XII 



ECHINODEBMS 



142. General characters. The division of the echino- 

 derms includes the starfishes, sea-urchins, serpent- or brittle- 

 stars, sea-cucumbers, and crinoids or sea-lilies. All are ma- 

 rine forms, and constitute a conspicuous portion of the 

 animals along almost any coast the world over. From 

 these shallow-water situations they extend to the greatest 

 depths of the ocean, and the bodily form possesses a great 

 number of variations, adapting them to lives under such 

 diverse conditions; and yet there is perhaps no group of 

 organisms so clearly defined or exhibiting so close a resem- 

 blance throughout. At one time it was thought that their 

 radial symmetry was an indication of a close relationship 

 to the coelenterates, but more careful study has shown them 

 to be much more highly developed than this latter group, 

 and widely separated from it. A skeleton is almost always 

 present, consisting of a number of calcareous plates embed- 

 ded in the body-wall, and often supporting numbers of pro- 

 tective spines, which fact has given to the group the name 

 Echinoderm, meaning hedgehog skin. 



143. External features. The body of a starfish (Fig. 90) 

 consists of a more or less clearly defined disk, from which 

 the arms, usually five in number, radiate like the spokes 

 of a wheel. At the center of the under side the mouth is 

 located, and from it a deep groove, filled with a mass of 

 tubular feet, extends to the tip of each arm. Innumerable 

 calcareous plates firmly embedded in the body-wall serve 



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