64 COELENTEKATA AND 



Ctenodiscus. Aboral surface paved with short, thickly 

 set spines. Madreporic body large. Central protuber- 

 ance in centre of aboral surface. Edges of rays paved 

 with rectangular plates which bear spines. Kays termi- 

 nated by a single median rounded plate. 



C. crispatus. 



Asterina. Body pentagonal, thick, flat with thin mar- 

 gin, destitute of rectangular plates. Small. 



A. borealis. 



Pteraster. Body with aboral region covered by a thin 

 tent-formed integument stretched over the body poised 

 upon the tips of clusters of aboral spines to which it is 

 joined. Tent-like membrane flexible. There is a central 

 opening in tent-like membrane. Madreporic body hidden 

 and seen by cutting off the covering. Young carried in 

 the groove-formed marsupia extending from the sexual 

 openings to the central orifice. P. militaris. 



Hippasterias. Body with short, stumpy spines. Bright 

 red color; obscurely pentagonal. Aboral plates of uni- 

 form size. H. phrygiana. 



OPHIUROIDEA. 



Stellate echinoderms with central disk and long, flex- 

 ible, simple or branched arms. Body markedly separate 

 from the arms. Ventral surface of the arms covered with 

 plates or integument. Stomach and ovaries confined to 

 the disk. No ambulacral suckers ; locomotion by spines, 

 hooks and motion of the arms. Viviparous, or young have 

 the form of pluteus. 



Rays simple, not branched, ventral plates present. 



Ophiuridae. 



Rays branched, ventral plates replaced by a leathery 

 skin. Arms capable of infolding about the mouth. 



Astrophytidae, 



