270 



THE STELLEROIDEA 



FIG. XXV. 



Ophiopyrgus, seen from the side (after Lyinan). 

 c, central, b, basal, /, radial plates. 



dorsal plates of Ophiuroids, that one or more pentamerous rings 

 should be more conspicuous than the rest. 



In the previous cases the dorsal plates of the disc are more pro- 

 minent than in Ophiura. In some other Ophiuroids, however, these 



plates are less important. 

 Thus the radial shields may 

 be buried beneath the in- 

 tegument, or may be absent 

 altogether, as in Neoplax ; 

 the remaining plates may 

 also be reduced to scales 

 or small granules embedded 

 in a soft integument, as in 

 the Gorgonocephalidae. 



Variations in the Arm 

 structure offer important 

 indications as to the affinity 

 between Ophiuroids and As- 

 teroids. The differences be- 

 tween such types as Ophiura, 

 and Asterias are very great ; 

 but many intermediate 

 stages occur between these 

 extremes ; and in the case of some extinct genera it is doubtful 

 to which sub-class they belong. 



The vertebral ossicles of some existing deep-sea Ophiuroids, such 

 as Ophiogeron (Fig. XXVI.) or Ophiohelus (Fig. XXVII.), consist of 

 two separate bars similar to the ambulacra! ossicles of Asteroids. The 

 same arrangement holds in some fossil species, such as Lapworthura 

 Miltoni. A further approximation to the arm struc- 

 ture of Asteroids occurs in some genera from which 

 the ventral arm-plates are absent. The radial water- 

 vascular vessels then lie along open grooves, as in the 

 Asteroids. This condition is found among recent 

 Ophiuroids in the genus Ophioteresis, and among fossil 

 members of the class in the extinct Lysophiurae. 



In such cases it is generally easy to recognise that 

 the lateral arm-plates are homologous with the adam- 

 bulacral plates of Asteroids, a point also well shown by 

 the living Opliioteresis (Fig. XIV.) 



The lateral arm-plates, though often smaller than 

 the ventral or dorsal arm-plates, are morphologically 

 the most important. They are always present, except when the 

 arm is covered by a soft or granular integument, as in the 

 Cladophiurae and some genera of Streptophiurae. 



The dorsal arm-plates vary more than either of the other sets. 



l.n.p 



FIG. XXVI. 



Arm -plates of 

 Ophiogeron mtfii- 

 nus (magnified). 

 n, ambulacra] 

 plate ; l.c.p, lat- 

 eral arm-plates ; 

 s, spine. 



