222 GENERAL ZOOLOGY 



arms with reference to the central region, or disk, is practi- 

 cally the same as the position of radii in a circle. Hence we 

 say that the starfish is radially symmetrical. We find it 

 necessary also to change a few terms of location. In the 

 picture of the tide-pool study we are looking down upon the 

 aboral surface of the three starfishes. The opposite side has 

 the mouth at the center, and for that reason is called the 

 oral surface. 



The aboral surface and part of the oral surface of Asterias 

 is thickly set with hard, limy spines that arise from small 

 plates of the same material just within the skin. Between 

 the spines there are many short, soft projections — the gills. 

 The spines of the oral surface are longer and more pointed 

 than those of the aboral surface. By examining the aboral 

 spines with a hand lens one can make out a circle of very mi- 

 nute structures surrounding the base. Each of these struc- 

 tures consists of a short stalk supporting two branches that 

 open and close like nippers. These short stalks with their 

 branches are called pedicellarise. The body surface is kept 

 clean by the pedicellarise, and small animals are prevented 

 from injuring the delicate gills by the protective action of 

 these same organs. 



Along the middle of the oral surface of each arm there is a 

 groove which begins at the mouth and ends near the tip. 

 The roof of the groove is formed by two series of flat, cal- 

 careous plates, set together like the rafters of a frame house. 

 Between the plates four rows of slender, flexible tube feet 

 (Fig. 121, ab) extend to the outside. The groove is called the 

 ambulacral groove, because it protects the ambulacral organs 

 (Lat. ambulare, "walk"). A few of the ambulacral organs 

 (tube feet) near the tips of the arms are sense organs for 

 "smelling" food. The remainder are the organs of locomo- 

 tion. At the very end of each arm there is a small red eye, 

 protected by a circle of spines. 



