102 



INVERTEBRATE ANIMALS. 



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

 ( Ophiurd) 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 Ophiuridce consists 

 of a circular central disk covered with small calcareous plates, 

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



Fio. 84. Ophiuroidea. a Ophiivra texturata, the common sand-star; I Ophiocoma 

 neylecta, 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 



