CHAPTER XII 

 THE ECHINODERMS 



In the Echinoderms, or spiny-skinned animals, Nature 

 has worked out a peculiar type of organization very dif- 

 ferent from what is found anywhere else in the animal 

 kingdom. One conspicuous feature of most echinoderms 

 is their apparently radial symmetry. The parts of the 

 body are arranged about a central axis instead of merely 

 on two sides of a median plane as in insects and vertebrates. 

 This radial structure led the older naturalists to class the 

 echinoderms along with the jelly-fish and their allies in a 

 group called Radiates, but it is now known that jelly-fish 

 and echinoderms are but very distantly related. Even 

 the radial structure on which the alliance was based is 

 now known to be secondary in the echinoderms and de- 

 rived from a primitive bilateral symmetry which can still 

 be traced in the position of certain organs of the body. 

 Without exception the echinoderms are confined to the sea. 



One large division of the Echinoderms consists of the 

 Asteroids, or starfishes, of which there are many species 

 of various forms and colors. Very commonly there are 

 five rays which extend from a central disk, but in some 

 cases there may be six, and in a few, even over twenty. 

 The body wall is hardened by a deposit of lime, but not to 

 such a degree as to prevent more or less movement of the 

 rays. The principal organs of locomotion are the tube 

 feet which project in rows through the under sides of the 

 rays where they are lodged in grooves. In most species 

 the tube feet end in adhesive suckers. By extending, at- 



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