296 



ECHINODERMA 



A line passing through the madreporite and the opposite arm divides the 

 body into symmetrical halves. This arm is called anterior, since in the irregular 

 sea urchins (Spatangoids) the homologous area is clearly anterior, while the 

 madreporic interradius is posterior. This plane of symmetry does not corre- 

 spond with that of the larva. The two rays on either side of the madreporite 

 form the bivium, the three others the trivium. 



FIG. 285. FIG. 286. 



FIG. 285. Comet form of Linckia mult (flora (from Korschelt-Heider) One of the 

 arms is producing'a new animal by budding. 



FIG. 286. Culcita pent angular is, aboral view (from Ludwig). a, madreporite; b, 

 reflexed end of ambulacral grooves. 



The skin is everywhere protected by large and small plates jointed 

 together. In life it is extremely flexible, the arms can be bent in any 

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

 Of the skeletal pieces the ambulacral plates need special mention. These 



FIG. 287. A, cross-section of starfish arm (orig.). o, adambulacral plates; am, 

 ambulacra; ap, ambulacral plates; b, branchiae; c, coelom; /?, hepatic caeca; i, inter- 

 ambulacral plates; n, radial nerve; p, ampulla; r, radial canal; 7', radial blood vessel. 

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



form the roofs of the ambulacral grooves, and between them are openings 

 (fig. 287, B), the ambulacral pores, through which connection is made be- 

 tween the ambulacra and ampullae. In each arm the pairs of ambulacral 

 plates meet above the groove like the rafters of a roof. Laterally each 



