CLASS OPHIUROIDEA 



BRITTLE-STARS 



ophiurans, or brittle-stars, differ from the starfishes in 

 JL having the arms quite distinct from the body. In starfishes 

 the arms are extensions of the body, and the viscera extend part- 

 way into them, while in ophiurans the body is a central disk, with 

 the five narrow arms attached to its margin. In form they sug- 

 gest a spider, and sometimes are called sea-spiders. The name 

 brittle-star is also descriptive, since they break off their limbs 

 readily. The arms are narrow, taper to a point, usually curl at 

 the ends, and have no ambulacral grooves, but are completely 

 covered with bare calcareous plates (except in the first order, where 

 they have a skin and no plates). The ambulacra are small spines 

 without suckers, which emanate from the sides or margins of the 

 arms and do not serve for walking. The madreporic plate is on 

 the ventral surface, in one of the circular shields which surround 

 the mouth. The alimentary canal ends blindly. The egg-sacs 

 lie between the arms and open by slits on the under side, close to 

 the arms, where they join the disk. Water flows in and out of 

 these pouches, which are thought to have also respiratory and 

 excretory functions. No eye-specks have been found, but they 

 must exist, since the animal is sensible to approaching danger and 

 quickly retreats. The ophiurans are more active than starfishes, 

 moving by wriggling, and clambering with their arms. They are 

 shy and hard to find, and it is difficult to capture a whole one, 

 since they throw off pieces of their arms at the least alarm. Often 

 they completely dismember themselves, an action which does them 

 no permanent injury, since they reproduce lost parts. They are 

 more commonly inhabitants of deep than of shallow waters, and 

 are brought up from the bottom in dredges in great numbers. 



214 



