ECHINODERMATA 



131 



The lower surface, usually attached, has a mouth, five grooves 

 (ambulacral) with two to four rows oi tube feet extending from them. 



The skeleton is made up of calcareous plates {ossicles) united by 

 connective tissue. Ossicles are regularly arranged around the 

 mouth and in the ambulacral grooves and often along the sides of the 



Fig. 55. Common starfish. (From Mayer. Courtesy of N. Y, Zool. Soc.) 



arms. The ambulacral ossicles are movably articulated so that they 

 can open or close the groove. At the end of the ray the ambulacral 

 ossicles end in a median terminal ossicle. There are thus two or 

 three rows of movable ambulacral spines. The spines are shoj-t and 

 blunt, covered with ectoderfu and arranged in irregular rows parallel 

 with the long axes of the rays. They are supported on irregularly 

 shaped ossicles. 



