XIV] OPHIUROIDEA 341 



nerve, and above this again is a large rounded disc of calcareous 

 matter, the so-called vertebra. This really corresponds to a pair 

 of ambulacral plates which have become fused together. So much 

 might be inferred from the fact that the radial water-tube runs in a 

 groove on its under surface, and it is clearly proved by examining 

 young specimens. Each vertebra is very short, and it not only 

 has rounded knobs and cups in order to enable it to slide on its 

 successor and predecessor, but is connected to each of them by four 

 great muscles, by the contraction of which the arm is moved in any 

 direction (Fig. 156). If the two side muscles contract the arm is 

 moved towards that side, if the two upper, upwards, and so on. 



FIG. 157. A diagrammatic vertical section of an Ophiuroid. After Ludwig. 

 The circumoral systems of organs are seen to the left, cut across, their 

 radial prolongations cut longitudinally, to the right. 



1. Body- wall. 2. Mouth. 3. Coelom. 3 1 . Coelom of the arm. 



4. Mouth papillae. 5. Torus angularis. 6. Oral plate. 7 1 . 1st am- 

 bulacral ossicle. 7 2 , 7 3 , 7 4 . 2nd to 4th ambulacral ossicle or "vertebrae." 

 8 1 , 8 2 , 8 3 . 1st to 3rd ventral plate. 9. 1st oral foot. 10. Trans- 



verse muscle of the 2nd joint. 10 1 . External interradial muscle. 



10 2 . Internal interradial muscle. (The line should point to the dotted 

 tissue.) 11. Water- vascular system : to the left the circumoral ring, 



to the right the radial vessel. 12. Polian vesicle. 13. Nerve-ring and 

 radial nerve: the ganglia on the latter are not shown. 14 1 . Genital 

 rachis lying in the aboral sinus. 15. Radial perihaemal canal. . 



These muscles are the seat of the chief activities of the animal, and 

 it is not surprising to find that a pair of large nerves comes off 

 between each two vertebrae to supply them, and that where these 

 nerves are given off the nerve-cord is thickened and the nerve-cells 

 increased, so that a string of ganglia is produced strongly recalling 

 the ventral nerve-cord of the Earthworm. Between the vertebrae 

 and the radial nerve-cord there is a single canal (2, Fig. 156), 

 representing the pair of radial perihaemal canals in a similar position 

 in Asteroidea. From the ventral wall of this canal the coelomic 

 nervous system is formed : and it is by the greater development of 

 this system where the nerves to the ambulacral muscles are given 



