248 



/ 'ff YL I \M E CHI NODE KM A . 



Class OPHIUROIDEA. Brittle-stars, e.g. Ophiopholis bellis. 



Echi7ioderms with a stellate flattened body, nearly related 

 to star-fishes, but usually differing from them in having the 

 arms (sometimes branched} sharply marked off from the 

 central disc, no ambulacral groove on the ventral surface of 

 the arms, the digestive organs and gonads restricted to the 

 disc, and the madreporite ventral. There is no anus. There 



are deep respiratory 

 clefts on the disc at 

 the insertion of the 

 arms. They agree 

 with star -fishes in 

 being free, in having 

 radially disposed 

 gonads, in having the 

 tube-feet restricted to 

 the under surface, 

 and in other features. 

 The body of 

 a brittle-star differs 

 from that of a star- 

 fish in the abrupt- 

 ness with which the 

 arms spring from the 

 central disc (cf. 



FIG. 118. Ventral surface of disc of an Brisin^d] These 



Ophiuroid (Ophiothrix fragilis). -- After nrp rmmrnlnr 



Ge^enbaur drills tire niuscuitir } 



., Openings of genital pockets or bursa ; i., aO( ^ USeful ill WHg- 

 mouth ; v., ventral plates of arms ; s/>. , spines of 

 arms ; tf., tube-feet, at the right side these are 

 represented as retracted ; <?., the openings through 

 which they are protruded ; /., plates around mouth 

 bearing the so-called teeth ; one of these plates is 

 perforated, and functions as the madreporite. 



and clamber- 



^-j ^^ 



ing ; they do not 

 contain outgrowths 

 of the gut, nor re- 

 productive organs. 



Moreover, there is no ambulacral groove, and the tube-feet 

 which project on the sides are too small to be of locomotor 

 service. Each segment of the arm includes a central 

 "vertebral ossicle," with four plates forming a tube round 

 about it. There is a complete oral skeleton. The madre- 

 poric plate is situated on the ventral surface, usually on one 

 of the plates around the mouth. The food canal ends blindly. 



