262 THE STELLEROIDEA 



the union of the bases of the arms, but is sharply marked off 

 from them, and they are attached to it as appendages ; there 

 is not, moreover, in the Ophiuroids the ventral groove of the star- 

 fish. These characters, however, are not absolutely to be relied 

 on ; thus in some species of Astroschema there is no sharp separa- 

 tion between the arms and the disc ; while in Ophioteresis (Fig. 

 XIV.) the radial ambulacral vessels and nerve-trunks lie in shallow 

 grooves on the ventral surface of the arms. 



An idea of the Structure of a typical OpMuroid may probably 

 be best obtained by the careful examination of a representative 

 species, for which purpose the commonest English brittle star 

 (Ophiura ciliaris, Linn, sp.) is a convenient type. 



This Ophiuroid consists of a round, flat, scale-covered disc, from 

 which radiate five long, tapering arms. The Arms are composed 

 of a series of jointed segments, each containing six plates. Two 

 of these are fused together into a single " vertebral ossicle," and 



Fir;. XIV. 



Ophioteresis (after Bell). Aboral surface of an arm ossie e ; rf, the double dorsal ariii-pMes ; 

 , articular cavities ; I, lateral arm-plates. 



a series of these forms the axis of the arm. The remaining four 

 plates form an external tube round the vertebral ossicle. One 

 pair occurs at the sides, and is known as the lateral arm-plates or 

 shields. Another of the four plates lies above the vertebral 

 ossicle, and is accordingly known as the dorsal arm -plate (or 

 dorsal shield) ; the fourth lies on the lower surface of the arm, 

 and is accordingly known as the ventral arm -plate (or ventral 

 shield). Each lateral arm-plate bears seven short spines. 



The plates forming the central chain of the arm are known 

 as " vertebral ossicles," because, in typical Ophiuroids such as 

 Ophiura ciliaris, they articulate by a series of knobs and sockets 

 like the bones of a vertebral column. In a typical vertebral 

 ossicle the two articular surfaces are very different ; in the proximal 

 or adoral surface (that nearer to the disc) the most conspicuous 

 features are the prominent central umbo (Fig. XV. u) and two broad 

 " lower muscle fields " (Fig. XV. l.m) at the two lower angles. 

 Above the umbo there is a narrow " upper canal furrow " (./), while 

 a corresponding " lower canal furrow " (l.f) occurs between the two 



