228 Description of the Plates. 



runs the gang-liated nerve-cord. Each ambulacral ossicle 

 abuts upon its fellow by a facet a little below the level to 

 which the lines /cj, k 2, are drawn, the interval between 

 this level and that of the articular facets being- filled up by 

 muscular fibres, which by contracting must act as diyari- 

 cators of the ambulacral ossicles. Adductor muscular fibres, 

 on the other hand, are found in the concavity of each ambu- 

 lacral arch, crossing from one ossicle to the other in the 

 interval between the radial water-vessel and the nerve-cord. 

 The structures in the Ophiuridae, which are homologous 

 with the ambulacral ossicles of the Asteriae, are the two 

 immoveably articulated halves of a vertebral ossicle, which, 

 in the absence of any prolongations of the digestive or 

 generative organs into the arms, occupies a much larger 

 space relatively than the ambulacral ossicles of the Asteriae. 

 The structiires in the Echinoidea, which are homologous 

 with the ambulacral ossicles of the Asteriae, are the so- 

 called 'Auriculae' of Echinus, which in Echinanthus are 

 repeated upon each ambulacral plate. The ambulacral 

 plates, again, of the Echinoidea are not represented by any 

 calcification in the Asteriae, being developed in the layer of 

 perisoma which lies externally to the radial nerve-cords, 

 and which remains free from induration in the order just 

 named ; though it carries four rows of scutes, one dorsal, 

 two lateral, and one ventral, in the Ophiurae. 



For the digestive system of the Asterias {AsteracantJdon) rubens, 

 see Miiller and Troschel, System der Asteriden, Taf. xi., fig. i, 

 p. 132. 



For the reproductive system, see Miiller and Troschel, ibid., pp. 



133. 134- 



For the motor and skeletal system, see Professor Sharpey, Cyclo- 

 paedia of Anatomy and Physiology, article ' Echinodermata,' 

 figs. 9 and 12, pp. 32, 34; Wilson, Linn. Soc. Trans., vol. xxiii., 

 i860, p. 107, tab. 14, fig. 7. 



For the skeletal system, see Gaudry, Ann. Sci. Nat., Ser. iii., tom. 

 xvi., 1851, pi. 3, fig. '• 



For the homology of the Auriculae of the Echinoidea and the cal- 

 careous ring of the Holothuroidea with the ambulacral ossicles 



