/. A8TEROIDEA. 



335 



tion, and the animal can work its way through narrow openings. 

 Of the skeletal pieces the ambulacral plates need special mention, 

 A B 



FIG. 312. -4, cross-section of starfish arm (orig.). a, adambulacral plates; am, ambu. 

 lacra; op, ambulacral plates; 6, branchiae; c, coelom: /i, hepatic caeca; i, inter- 

 ambulacral plates; n,, radial nerve; p, ampulla; r, radial canal; r, radial blood- 

 vessel. B, ambulacral plates, ventral view, showing the ambulacral pores 

 between. 



These form the roofs of the ambulacral grooves, and between them 

 are openings, the ambulacral pores, through which connexion is 

 made between the ambulacra and ampullae. In each arm the pairs 

 of ambulacral plates meet above the groove like the rafters of a 



FIG. 



313. Asteriscus verruculatus, aboral surface removed. (After Gegenbaur.) 

 gonads ; /i, hepatic caeca ; /, stomach with anus. 



roof. Laterally each ambulacral plate abuts against a small inter- 

 ambulacral plate, bearing usually movable spines. Beyond these 

 comes the adambulacral plates, and then those of the aboral sur- 

 face. Each ambulacral area terminates at the tip of the arm with, 

 an unpaired (ocular) plate. 



