STARFISH AND OTHER ECHINODERMS 363 



. External Anatomy 



The body is composed of a central disc and some (usually five) 

 radiating arms or rays. The mouth is located in the center of the 

 under or oral surface while the upper or aboral surface is covered 

 with spines of various lengths. On the arms these spines are ar- 

 ranged somewhat in rows. Between the spines the exposed skin 

 is extended into projections known as papula or dermal branchiae. 

 There are some small pincherlike structures, called pedicellariae, 

 arranged around the bases of the spines, which serve to keep the 

 surface of the exposed papulae clear of debris and foreign material. 

 The pedicellariae are composed of two jaws or blades and a basal 

 plate with which the jaws articulate. There are large and small 

 pedicellariae. In an eccentric position on the aboral side of the 

 central disc is found the calcareous, sievelike madreporite. The 

 portion of the central disc and two rays adjacent to the madreporite 

 constitute the bivium. The other three arms and their adjacent por- 

 tions of the central disc compose the trivium. On the oral side sur- 

 rounding the mouth is a perioral membrane or peristome. An ambu- 

 lacral groove, containing rows of tube feet, radiates from this along 

 the oral side of each arm. A reddish pigment spot in the end of 

 each arm is called an eye. The spines are longer and stronger around 

 the mouth and along the margins of the ambulacral grooves than 

 elsewhere. 



Internal Anatomy 



The body wall is relatively strong and hard without being per- 

 fectly rigid. This condition is due to the presence of the calcareous 

 skeletal plates throughout, which are bound together by connective 

 tissue and muscular fibers. These plates are often called ossicles. 

 They lie in a flat position in the aboral portions of the body wall. 

 The skeleton of the ambulacral grooves consists of four rows of 

 articulated, oblong ossicles in each arm. These ossicles are arranged 

 with the flat sides together, like cards in a filing case. The two 

 middle rows of ossicles are called ambulacral plates. Ambulacral 

 pores, through which the tube feet project, are located between these 

 plates. The outer rows of plates, forming the margin of the groove, 

 are shorter and are known as adambulacral plates. Five flat oral 

 ossicles surround the mouth. 



Within the body wall and extending into the arms is a large coelom 

 which is lined by a peritoneum and filled with coelomic fluid. In 



