102 



INVERTEBRATE ANIMALS. 



dos, form). — In this order we have only the common sand-stars 

 ( Ophiura) and brittle-stars ( Ophiocoma), all closely allied to 

 the true star-fishes in external appearance, especially in their 

 strikingly radiate form. The body in the Ophiuridm consists 

 of a circular central disk covered vpith small calcareous plates, 

 and giving off five long, slender, snake-like arms (Fig. 34, a, b), 

 ■which may be simple or branched, but which do not contain 

 any prolongations from the stomach, nor have their under 

 surfaces excavated into grooves for the protrusion of ambu- 

 lacral tube-feet. The arms, in fact, are not prolongations or 

 lobes derived from the body itself, but are special appendages 

 added for purposes of locomotion and prehension. The arms 



Fig. 84. — Ophiuroidea. a Ophiwra texturata, the common sand-star; & Ophiocoma 

 ne{/l6Cta^ the gray brittle-star (after Forbes). 



are very much longer than the diameter of the disk, and are 

 protected by four rows of calcareous plates — one above, one 

 below, and one on each side. In the centre of each arm is a 

 row of calcareous pieces which form a kind of internal axis 



