48o 



ECHINODERMATA OPHIUROIDEA 



ventral and lateral plates, guarded each by a valve-like plate 

 called the "tentacle -scale." In Ophiothrix they are covered 

 with sense-organs, each consisting of a hillock -like elevation of 

 the ectoderm, in which are cells carrying long stiff sense-hairs. 

 In most Ophiuroids such organs are not present, though 

 abundant scattered sense-cells occur, and the outer surface of 

 the tube-feet and the lining of certain pockets called " genital 

 bursae" (Fig. 208, c/.b) are the only portions of the surface 

 where the ectoderm persists. Everywhere else, although present 



lateral arm 



upper arm plate. 



vertetra. 



ect. ■• • 



tentacle ,^ 

 scales. 



Ij ; nerv.rad. 



gang. p. 



underarm '. .. 

 plate. perih. 



Fig, 209. — Diagrammatic transverse section of the arm of an Ophiuroid. coe, Dorsal 

 coeloniic canal ; ect, ectoderm covering the tube-foot ; ep, epineural canal ; gang.p, 

 pedal ganglion ; L, nerve-cord ; muse, longitudinal muscles attaching one vertebra 

 to the next; nerv.rad, radial nerve -oord_; perih, radial perihaemal canal; pod, 

 podium (tube-foot) ; sp, lateral spines ; lo.v.r, radial water-vascular canal. 



in the young, it disappears, leaving as remnants a few nuclei 

 here and there attached to the under side of the cuticle.^ 



The greater part of the section of the arm is occupied by a 

 disc-like ossicle called the "vertebra." Each vertebra articulates 

 with its predecessor and successor by cup-and-ball joints, and it 

 is connected to each of them by four powerful longitudinal 

 muscles. Above, its outline is notched by a groove, in which 

 lies an extension of the coelom of the disc (Fig. 209, coe), but 

 contains no outgrowth of the alimentary canal, as is the case in 

 Asteroidea. The vertebra is also grooved below, and in this 

 lower groove are contained the radial water - vascular canal 



^ In the more primitive Opliiuroidea (Streptophiurae) it persists all over the 

 body ; in Cladophiurae it is found on the central part of the disc. ■ 



