CLASS OPHIUROIDEA 
BRITTLE-STARS 
HE ophiurans, or brittle-stars, differ from the starfishes in 
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 askin 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. 
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