OPHIUROIDEA 



145 



Its bathymetrical distribution is from the littoral region down to 

 ca. 180 m. 



III. Class. BRITTLE-STARS or OPHIUROIDS 



(Ophiuroidea) 



Free-living echinoderms with a round, flat, disk-shaped body 

 from which proceed 5 (rarely more) thin, articulate arms, well 

 marked off from the disk. Neither intestinal nor, as a rule, genital 

 organs pass into the arms. No open furrow along the ventral 

 side of the arms.^ Tube-feet in small pores in two series along the 

 under-side of arms. Mouth turning downwards. Intestinal organs 

 a simple sac-shaped stomach, without caeca or anal opening. 



Fig. 83. — A Brittle-star {Ophlocouiina nigra), dorsal (left figure) and ventral 

 side (right figure). Nat. size. (From Danmark's Fauna.) 



The body of the Ophiuroids (Fig. 83) is small, disk-shaped, 

 rarely exceeding a pair of centimetres in diameter ; only in 

 Gorgonocephalus it may be up to ca. 10 cm. in diameter. The 

 arms, which are always well defined against the disk, are long, 

 often many times exceeding the diameter of the disk, slender, as a 

 rule very flexible. A few forms of the order of the Euryalids have 



^ Only the palaeozoic Ophiuroids had an open ambulacral furrow. It 

 has been maintained that a recent form, Ophioteresis Bell, is lacking ventral 

 plates, thus being a very primitive form, related to the palaeozoic Ophiurids 

 with open ambulacral furrow, and this form has played an important role 

 in classification. But it is only a gross mistake ; the form in question has in 

 reality well -developed ventral plates and is nothing but the well-known 

 Ophiothela tigris Lyman of the family Ophiotrichidae. 



L 



