484 ECHINODERMATA OPHIUROIDEA chap. 



complete vertebra in the arm they can be rotated downwards 

 so as greatly to enlarge the mouth, and again rotated upwards 

 and inwards, when they form an excellent strainer to prevent 

 the entrance of coarse particles. To permit this extensive 

 movement the articulatory facets on tlie proximal surface of the 

 first vertebra have been much modified ; the median knob and 

 pit have disappeared, and the dorso-lateral pits are raised on to 

 the surface of processes, so that there are in all four processes, 

 two of which articulate with one half of a jaw. 



The mouth can be narrowed and the jaws forced inwards 



A? 



A^-' 



}. 212. — Lateral view of 

 moutli - frame of Ophi- 

 arachna incrassata. x 4. 

 A^l, peristomial plate, 

 possibly the half of the 

 tirst vertebra ; .4-, the 

 half of the second verte- 

 bra ; .4^, the third verte- 

 bra ; /'"', pores for pair 

 of tentacles ; gen, genital 

 scale lying beside open- 

 ing of genital bursa ; 

 muse, longitudinal nuiscles 

 connecting vertebrae ; 7i.r, 

 groove for nerve-ring ; ^j, 

 tooth ; ^J^, mouth-papilla ; 

 t, torus angularis. (After 

 Ludwig.) 



towards the centre hy the simultaneous contraction of five 

 muscles (ynusc. tr, Fig. 213) each, which unite the two halves of 

 a jaw. 



Turning now to the skeleton of the disc, we notice that 

 dorsally it consists of a closely -fitting mosaic of small plates, 

 which are usually concealed from view by a covering of minute 

 spines. Opposite the insertion of each arm there are, however, 

 a pair of large triangular plates (" radials "), which extend out- 

 wards to the periphery and strengthen it, much as the ribs do in 

 an umbrella. These radial plates are always exposed, in Ophio- 

 tlivix, even when the rest of the dorsal plates are concealed by 

 spines. On the under surface there is a similar plating ; but 

 adjoining the jaws are five large, more or less rhomboidal, plates. 



