XVII 



OPHIO THRIX VERTEBRAE 



481 



(Fig. 209, w.'y.r),and below it perihaemal canals as in Asteroidea ; 

 below this again the radial nerve -cord (Z), and beneath this 

 again a canal called the " epineural canal " («p), which represents 

 the missing ambulacral groove. This canal in the very young 

 Brittle Star is an open groove, but becomes closed by the 

 approximation of its edges. The vertebra, which has a double 

 origin, represents a pair of fused ambulacral ossicles. In 

 Ophiohelus these are only slightly adherent to one another 

 (Fig. 216). 



Flo. 210. Proximal and distal views of the three types of vertebra found amongst 



" Ophiuroidea. A, Ophioteresis, a type of the Streptophiurae (after Bell), x 24 ; 

 B, Astroschema, a type of the Cladophiurae (after Lyman), x 10 ; C, Ophiarachna, 

 a type of the Zygophinrae (after Ludwig), x 3. The upper figure in all cases 

 represents the distal aspect, the lower the proximal aspect of the vertebra, ii.g. 

 Ventral groove. 



When the surface of a vertebra is examined it is found that 

 it can be divided into a thin border, to which are attached the 

 four muscles by which it is connected to its successor and pre- 

 decessor, and a central portion, on which are situated the 

 knobs and pits, by means of which it articulates with the next 



vertebra. 



The simultaneous contraction of the two upper muscles 

 causes the arm to bend upwards. The contraction of the two 

 lower bend it downwards, whilst a sideward movement is effected 

 by the contraction of the upper and lower muscle of the same 

 side. On the proximal surface of the central portion of the 

 vertebra there is a central knob and two ventro-lateral knobs, 



VOL. I ' " 2 ^ 



