268 



THE STELLEROIDEA 



Ophiura. The most important addition to the plates found in that 

 genus is formed by the plates of the so-called "apical system." 

 In some species, such as Ophiomusium validum, there is a central plate 



br.c: d.s 



-pd. 



FIG. XXI. 



Diagrammatic section through arm of Ophiuroicl. br.c, dorsal branch of brachial coelomic 

 system ; d.s, dorsal arm-plate ; c.r.n, external radial nerve ; i.r.n, internal radial nerve ; Ls, 

 lateral arm-plate ; pd, podion ; pd.n, podial nerve ; r.e.c, radial epineural canal, and r.p.c, radial 

 pseudhaemal canal, together forming ventral branch of the brachial coelomic system ; r.r, radial 

 vessel ; v.o, vertebral ossicle ; r.s, ventral arm-plate. 



surrounded by three circlets, each of five plates ; these were once 

 regarded as homologous with plates in Asteroids and Crinoids, and 

 therefore named the infra -basals, basals, and radials. In some 

 species, such as Ophiomitra exigua, the two circlets of radially situated 



plates are absent, and only the 

 central and basal plates are 

 present. In Ophiocrene and 

 Ophiura convexa, Lym. sp., there 

 are only the central plate and 

 the radials; in Ophiura minuta, 

 Lym. sp., there are the central, 

 radial, and basal plates. The 

 last possible combination occurs 

 in a species of Pedinura (Fig. 

 XXII.), in which the two circles 

 of plates round the central have 

 been described as the radials 

 and inf ra-basals. In some other 

 forms, such as Ophioceramis ob- 

 stricta, Lym. (Fig. XXIII.), there 

 is a remarkable uniformity in the 

 plates on the dorsal aspect, but 

 these are not arranged on the 

 calicular plan. In some other cases, such as Ophiura inornata, Lam. 

 sp. (Fig. XXIV.), there are two "infra-basal" plates side by side 



FIG. XXII. 

 Dorsal plates of Fectinura, sp. (After Bell.) 



