1^2 



AN INTRODUCTION TO ZOOLOGY. 



becomes fixed, and the outer part is lost, setting free 

 tHe crinoid j this, however, is only fixed for a time, 

 the adult form being able to move about in a slow 

 way, by movement of its arms. 



The Starfishes, or Asteroidea, have a star-shaped 

 body, usually with five rays, but sometimes more. 



Fig. 43.— A Starfish, ^stropccieit i i-r«gula rts (reduced) : m, madreporiform 

 tubercle, which marks the centre of the bivium, viz. the two raya on each 

 side of it. The three remaining rays form the trivium. 



From the traces of bilateral symmetry shown by the 

 animal, these are sometimes spoken of as divided into 

 two sets, the bivium and trivium. The "feet" 

 are on the lower (oral) surface only, and they have 

 a skeleton of hard pieces protecting themj and on 



