viii ECHINODERMATA— MORPHOLOGY OF SKELETON 351 



III. Asteroidea. 



Here also the perisomatic portion forms by far the greater part 

 of the whole skeleton. Only in a few forms does the apical system 

 constitute a distinctly appreciable element in the skeleton. Further, 

 the oral system also, even if we include, besides the orals (odonto- 

 phores, proximal plates of the interbrachial system), the terminals, as 

 radials belonging to the oral system, forms but a very small fraction 

 of the whole skeleton. 



The skeleton of the Asteroidea is distinguished from that of most 

 Ecliinoidea by its mobility. It is not a rigid capsule, but its principal 

 plates are articulated one with another, and are movable one upon 

 another by means of muscles. The arms can bend upwards and 

 downwards, and also occasionally, to a certain degree, laterally (in the 

 horizontal plane). The ambulacral furrows may be deep, or shallow. 

 The disc is sometimes shortened in the direction of the principal axis, 

 i.e. flattened. 



In the perisomatic skeleton of the Asteroidea three principal parts 

 may be distinguished : (1) the ambulacral, (2) the interambulacral, 

 and (3) the accessory. 



(a) The Ambulacral Skeleton. 



From the free end, or tip, of each arm or ray a large median groove 

 runs on the oral side to the centre of the disc, and here runs into the 



Fio. 309.— Transverse section through the hrachial skeleton of Astropecten aurantia- 

 cus (Gray) ; original. For lettering see p. 317. sa, Supports of tlie ambulacral plates or supra- 

 ambulacral plates ; ad, adambulacral plates ; j), paxilla; ; 1, position of the radial canal, etc. ; 

 2, ampullje ; 3, ambulacral feet. 



mouth. In the base of this ambulacral furrow rise the ambulacral, 

 or tube-feet in two or four longitudinal rows (Figs. 239, 243, pp. 296, 

 298, and 343, p. 396). The plates of the ambulacral skeleton, which 



