THE STELLEROIDEA 



271 



They are double in Ophioteresis and Ophiomyxa pentagona, are split up 

 into several pieces in Astroschema (Fig. XXVIII.), and into a mosaic 

 of small plates in Hemieuryale ; or they are absent altogether from 

 many genera, such as Neoplax, Ophiobyrsa, Protaster, Lapworthura. 



d.a.p. 



La.p 



Fio. XXVII. 



Anu-plates of Ophiohelns umbrella, l.a.p, lateral anu-plates ; d.a.p, dorsal arm-plates ; 

 a, ambulacral plate ; s, spines. 



Another character by which the recent Asteroids and Ophiuroids 

 differ is the alternation of the ambulacral ossicles in the former, 

 whereas in the latter the pairs are opposite. The Palaeozoic 

 genera, however, also bridge this gap, as 

 in the order Lysophiurae the ambulacral 

 ossicles are alternate as in the Asteroids. 

 This is shown in the arm structure of 

 Palaeophiura. 



While Protaster and its allies on the 

 one hand approximate to the Asteroids, 

 another group of Ophiuroids has arms which 

 appear more like those of Crinoids. In 

 Ophiura the arms are fairly straight and 

 only capable of a limited movement in a 

 horizontal plane, for the vertebral ossicles 

 are " zygospondyline," i.e. are attached to 

 one another by a series of knobs, closely 

 fitting into sockets (e.g. Fig. XV.) In the group of the Strepto- 

 phiurae much more play of the arms is possible, as the ossicles are 

 " streptospondyline," i.e. articulate by simple ball-and-socket joints, 



FIG. XXVIII. 



The external arm-plates of a 

 segment of Astroschema (after 

 Lyman). v.o, ventral arm- 

 plates; v.a.p, lateral arm-plates; 

 the remaining six plates re- 

 present the dorsal arm-plates. 



