206 



ECHINODERMA 



A line passing througli tlie 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 

 madrejjoric 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 hivium, the three others the irivium. 



Fig. 285. Fig. 2S6. 



Fig. 2S5. — Comet form of Linckiamultiflora (f rem Korschelt-Heider) One of the 

 arms is producing a new animal by budding. 



Fig. 286. — Cukita pentangularjs, aboral view (from Ludwig). a, madreporite; 6, 

 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 amhulacral plates need special mention. These 



Fig. 287.— .4, cross-secUon of starfish arm (orig.). a. adambulacral plates; am, 

 ambulacra; ap, ambulacral plates; b, branchice; c, cctlom; /;, hepatic ca:ca; 1, inter- 

 arabulacral plates; )/, radial nerve; p. ampulla; r, radial canal; i\ radial blood'vessel. 

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



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

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

 tween the ambulacra and ampulla;. In each arm the pairs of amindacral 

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



