viii EGHINODEBMATA— MORPHOLOGY OF SKELETON 355 



coelomic vesicle. They must thus belong to the oral system, and 

 perhaps, in this system, correspond with the radials in the apical 

 system. 



In the development of the Asteroidea the formative centre of each 

 newly appearing plate in a radius of the perisomatic system is always 

 immediately proximal to the ocular plate of the arm. At these 

 points new plates continually appear between those last formed and the 

 ocular plates, Avhich thus alwaj's remain at the free tips of the arms. 



{(l) Comparison of the Perisomatic Skeleton of the Asteroidea with that of 

 the Echinoidea. 



The ocular plates (terminals) of the Asteroidea bear to the newly appearing 

 plates of the perisomatic skeleton relations altogether similar to those which the 

 radials (also "oculars") of the apical system in the Echinoidea bear specially to the 

 ambulacral plates. Since it has not been proved that the radial plates of the 

 Echinoidea arise over the right coelomic vesicle, it is possible that they, although 

 lying high up at the apex, belong genetically to the oral system, and correspond 

 with the terminals of the Asteroidea. The radials should then not be represented in 

 the apical system of the Echinoidea. 



In a comparison of the skeletons of the Echinoidea and the Asteroidea we should 

 then have to suppose that in the former the ambulacra have been lengthened round 

 over the ambitus to the apex ; and that, further, the body took on the form 

 of a pentagonal pyramid, by the abbreviation of the arms and the elongation of 

 the principal axis of the body ; and that, therefore, the whole region occupied by 

 the accessory skeleton of the Asteroid has disappeared. The marginal plates of the 

 Asteroid would then correspond with the interambulacral plates of the Echinoid, 

 and the adambulacral ossicles of the former with the ambulacral plates of the latter. 

 A comparison of the ambulacral plates of the Echinoid with the plates of the same 

 name in the Asteroid is rendered difficult by the diti'erence of position of the two, 

 the former being superficial, epiambulacral, and epineural, and the latter deeper, 

 subambulacral, and subneural. The ambulacral ossicles of the Asteroid would thus 

 not be represented in the skeleton of the Echinoid. 



IV. Ophiuroidea. 

 («) Skeleton of the Arms. 



The brachial skeleton of the Ophiuroidea consists typically of six 

 longitudinal rows of plates, a dorsal row (dorsal shields), a ventral 

 row (ventral shields), two lateral rows (lateral shields), and a double 

 row of internal ossicles lying in the axis of the arm. This system is 

 jointed, or segmented, in quite a regular manner — one dorsal, one 

 ventral, one axial piece and two lateral pieces together composing 

 a skeletal segment (Fig. 311). 



The external pieces together form, in each arm, a jointed tube, 

 which determines the shape of the arm. Most of the lateral shields 

 carry spines ; on each shield there are usually four of these, one 

 above the other, so that each longitudinal row of shields is armed 

 with four longitudinal rows of spines. The tube -feet emerge at 



