OPHIUROIDEA 



147 



by a thick, soft skin, which entirely conceals the plates. To the 

 outer edge of the lateral plates the arm sjnnes are articulated ; as 

 a rule they are more or less erect, outstanding, or (in the Eurya- 

 lids) directed downwards, but they may be closely appressed, in 

 which latter case they are often quite rudimentary ; in some 

 forms they assume the shape of hooks, especially the lower ones. 

 In other (deep-sea) forms they partly assume a very elaborate, 



Fig. 84. — A Brittle-star {Ophiura robusta) from the oral side ; at the upper 

 edge of the figure part of dorsal side of disk and arm of another species 

 {Ophiura affinis) ; enlarged. (From Danmark's Fauna.) 



d, Dorsal plate ; fp, Foot papillae or tentacle scales ; sp, Genital plate ; !?pa, Genital 

 papilla) ; gs, Genital or bursal slit ; ip, Inner comb of papillae ; k, Jaw ; m, Mouth shield ; 

 ma, Madreporite ; mp, Mouth papillae ; p, Arm spines ; pa, Outer comb of papillae ; po, 

 Pore of tube-foot ; po, Second foot pore ; r, Radial shield ; s, Side or lateral plate ; sm, 

 Adoral shield ; t, Teeth ; v, Ventral plate ; v,, First ventral plate. 



umbrella -like shape ; but they are never transformed into 

 pedicellarise (as has been erroneously maintained), these organs 

 being entirely unknown in the Ophiuroids. 



In the corners between the ventral and the lateral plates are 

 situated the pores through which the tube-feet or " tentacles " 

 issue, one pair corresponding to each arm joint. On the edge of 

 the pores some small papillae, the tentacle scales , are often found. 



