66 ECHINODERMATA. ASTEROIDEA. 



vessel. This surface then is the oral, ambulacral, or 

 ventral. The upper surface is aboral, anti- ambulacral, or 

 dorsal ; on it occurs the anus, a small opening placed near 

 the centre of the disc. In some forms the anus is absent. 

 Also on the dorsal surface between two of the rays there 

 is a porous plate, the madreporic tubercle; sometimes more 

 than one is present. 



On the ventral surface of each arm, there is a deep 

 longitudinal groove, the ambulacral groove ; this is formed 

 by two rows of plates known as the ambulacral ossicles, 

 (fig. 15 a) which are inclined to each other so as to form 



b e e 



FIG. 15. Section of the arm of a star-fish (Astropecten). a, ambulacral 

 ossicles; b, adambulacral plates; c, inferior marginal plates with 

 spines ; d, superior marginals ; e, radial water vessel : /, ampulla ; 

 <7, tube-feet. 



an obtuse angle ; the ossicles on opposite sides of the groove 

 are movably articulated at their dorsal surfaces. Along 

 each row of ambulacral ossicles, there is a row of pores, 

 each pore coming between two plates and being formed by 

 the apposition of two notches on the oral and distal faces 

 of adjoining plates ; generally the pores are arranged in a 

 straight line, but in some forms as in the common star- 

 fish, Asterias rubens, they are 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 

 adambulacral plates (b). In some forms beyond this row, 



