108 ECHINODERMA. ASTEROIDEA 



of tubular processes, the tube-feet, which are connected 

 with the radial water- vessel. This surface of the body is 

 called the oral, ambulacral, actinal, or ventral surface. The 

 upper surface is aboral, anti-ambulacral, abactinal, or 

 dorsal ; on it usually occur the anus and madreporic plate. 

 The anus is situated near the centre of the disc, but in 

 some forms {Astropecten) is absent. The madreporite or 

 madreporic plate is a porous plate placed between two of 

 the rays on the disc ; sometimes more than one is present. 

 On the oral surface, extending from the mouth to the 

 tip of each arm, is a deep groove, the ambulacral groove ; 

 this is formed by two rows of plates known as the 

 ambulacral ossicles (fig. 42, a), f a a f 



which meet at an angle ; the 

 ossicles on one side of the 

 groove are movably articulated 

 with those opposite. Along 

 each side of the ambulacral ^. 



Eig. 42. Section of the arm 

 groove there IS a row of pores, of a star-fish (Astropecten). 



each pore being between two «Lt m 1 bulac 1 ra ! ° f ssicles ; - h ad_ 



r & ambulacral plates ; c, mfero- 



OSsicles ; generally the pores marginal plates with spines ; 



■i • , • I , v d, supero-marginals ; e, radial 



are arranged in a straight line, wate £ vessel .^ ampu ' lla . g% 

 but in some forms they are tube-feet. Enlarged, 

 zigzag, being alternately near to, and distant from, the 

 middle of the ambulacral groove. External to the rows of 

 ambulacral ossicles there is, on each side of the groove, 

 another row of plates, the ad-ambulacral plates (b). At 

 the sides of the arms there are in many cases two rows of 

 marginal plates (c, d) — the super o- and infer o-marginals. 

 In some forms marginal plates are small or absent. The 

 aboral surface of the skeleton is often formed of a mesh- 

 work of calcareous rods with leathery skin in the inter- 

 spaces, but in some forms rows of distinct plates extend 



