CHAPTER XVII 



ECHINODEEMATA {CONTINUED): OPHIUKOIDEA = BRITTLE STAES 



CLASS 11. OPHIUKOIDEA 



The second class .of Eleutherozoa are familiarly known as 

 " Brittle Stars," on account of their tendency, when seized, to 

 escape by snapping off an arm, although this habit is by no 

 means confined to them, but is shared in a marked degree by 

 many Asteroidea, siich as Luidia, for instance. Like the 

 Asteroidea, they are " starfish," that is to say, they consist of 

 a disc and of arms radiating from it ; but the scientific name 

 Ophiuroidea really expresses the great dominating feature of 

 their organisation. Literally it signifies " Snake-tail " (o<^t?, 

 snake ; ovpd, tail), and thus vividly describes the wriggling, 

 writhing movements of the long thin arms, by means of which 

 the Ophiuroid climbs in and out of the crevices between the 

 stones and gravel in which it lives. This feature, viz. the 

 effecting of movement by means of muscular jerks of the arms, 

 instead of by the slow protrusion and retraction of the tube- 

 feet, is the key to the understanding of most of the points 

 wherein the Brittle Stars differ from the true Starfish. 



Asteroidea and Ophiuroidea agree in the common ground- 

 plan of their structure, that is, they both possess arms ; but 

 the most obvious difference in their outer appearance is that 

 whereas in Asteroidea the arms merge insensibly into the disc, 

 in Ophiuroidea the disc is circular in outline and is sharply 

 marked off from the arms. Closer inspection shows that in the 

 Ophiuroid the arms are continued inwards along grooves, which 

 run on the under surface of the disc, and that they finally 

 coalesce to form a buccal framework surrounding the mouth. In 



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